Rocker Arm Geometry

Rocker Geometry JIM MILLER

ROCKER ARM GEOMETRY seems to is wrong with what they were doing and lengthening itself as it moves across the raise its head every now and then, and then begin to explain what they needed valve tip, and you see this by the wide when it does, I rarely ever see it stated to change. At that point I could break foot print (we call a “witness mark”) accurately. Too often a sound bite of only down the simple rules for what geometry atop the valve tip. a small piece of information is taken out really is, and why. The use of rocker arms goes back to of context and then used as the Gospel, many things predating engines, but the totally ignoring the other dynamics Background of Rocker Geometry principles were never required to be so that revolve around it. In some cases, Rocker geometry (or the lack of it), speciﬁc on axis point heights and their something totally erroneous is stated that goes way back to many fathers on both consequences, as it is for helicopter bell is not only wrong, but makes no sense sides of the ocean, to when the Wright cranks, and racing engines! There was for anyone who just stops and thinks Brothers were still studying the theories no rocket science to designing these about it objectively. of lift in an airfoil. But for our purposes parts a hundred years ago, which ended When lecturing at trade shows, here, and to avoid boring the curious up on our prehistoric cars and early schools, engine shops or just getting who’ve managed to get this far on this airplanes. Engineers simply made designs pinned down on the phone by a story, I’ll come to the point about rocker that tried to minimize the degree of how knowledgeable engine builder going arms and explain as needed how the much scufﬁng was imposed on the valve deeper than most on a technical “mistakes” got to where they did. tip; got it close, and moved on to more issue, I have found that I spend about In the old days, rocker arms were important questions. Somewhere along half my time trying to undo various all pretty much what we term a “shoe” the line, there became a principle to get misconceptions about rocker geometry design; meaning the contact pad with the this in a general ballpark, that someone before I ever begin explaining the facts. valve had a large radius scruff surface later coined as the 1/3-2/3 theory (or There has been so much info put out that depressed upon the valve tip as either of the two). This placed the pivot there by reputable companies (and by the rocker moved through its rotation. point of the rocker arm so that it was my reckoning, incorrect), that people are The term of course comes from the 2/3 of the way below the valve tip, or the reluctant, by nature, to see something appearance to a shoe’s sole, but also to valve tip was 1/3 of the way above the different from the prejudice of what the mechanical motion much like a foot rocker shaft, depending on your point of they already know or think they know. and boot would do, as it pushes off. view. But the answer was the same. This If people are used to doing something a This pushing off motion, as many will thinking was originally derived from the certain way, they see everything from that already know, has the effect of the rocker intention that a near 90 degree arc could perspective. Usually, my getting through arm stretching itself as it moves through be realized when the valve reached its to them involves discrediting what I think the depressing (lift) cycle. It is actually intended full open position. Bear in mind

20 APR-JUN 2010 engine professional that valve lifts back then were usually drawing a roller tappet in various stages pad. So when his rocker was placed on in the quarter inch or so range, on little of lift as the cam lobe goes around to the engine, and the roller was positioned two and four cylinder engines. So being push upon it, and see a straight line from for a good “eye-ball” track on the valve, off a little really had no measurable its axis to the cam lobe is constantly now the push-rod cup was too high. difference in performance of the engine, shifting around as the tappet goes from The result, was that it went way up and and wear and tear was the real yardstick the close (base circle) position, up along in toward the stud. In actuality though, engineering back at the turn of the the acceleration ramps, then over the most engine builders in the sixties century was aimed at. Also, the ability nose. When finally, as it crosses dead continued to keep using standard length to accurately measure wear and tear, center at full LOBE lift, this straight line pushrods, and the excessive motion from horsepower, thermal loss and many between its axis and the cam centerline this mistake was occurring on the valve other cool things we take for granted is also in alignment with the tappet tip, which was deemed “normal” because on today’s computers, wasn’t even a bore itself. At all other times, the tappet the roller rolled! Believe it or not, even possibility back then. is actually receiving some level of side to this day, people think that the roller The advent of more valve lift, thrust in its bore (engine block) from the tip is for rolling on the valve. It is NOT. and thus pushing a budding internal pushing out forces that the lobe imposes The roller tip is for one reason only, and combustion engine technology higher to as it chases it up. that is to convert the shifting length of produce more power was really inspired Even when engineers were chasing the rocker’s arc (that moves across the for leaps and bounds by the advent of efficiency with aircraft engine and power valve on a shoe design), to a fixed length aviation, not Henry Ford. Not to take development, the threshold for seeing that moves far less in its effect, because anything away from the automotive measurable loss of engine life, like valve it is always point down in line with the crowd’s contributions, but only aviation stem or valve guide wear, was not easy valve’s motion, just as a roller tappet of a imposed the second requirement that unless things were really out of whack. cam is always aiming its contact tangent defined “efficiency” – and that was They weren’t worried about loss of cam line with the axis of the camshaft. light weight. Making Goliath engines events through small changes in rocker This error stayed, and was copied by that had more power was a lot easier geometry, even though they knew the many manufacturers and eventually by than making more power from light variables existed. So if they kept to this everyone in some measure or another. weight engines that would be flying over 1/3-2/3 rule, everything looked good It would take several decades before somebody’s head, somewhere. So the on the valve tip, and the leverage of the enough trial and error, and even a patent whole “thinking process” for efficiency rocker arm upon the spring – or more would be studied to make manufacturers in engine technology really found its accurately stated – the leverage of the rethink this, slowly improving their impetus in aviation, because racing back spring on the rocker arm was at near designs. Ironically, some of the most in the early 1900s was still done on the perpendicular relationship with the valve well known names continue to promote back (or behind) of one-horsepower when rates were at their highest. Even designs they never changed, and even whose exhaust was more easily stepped though by today’s standards for valve promote the less accurate means of using in than emitted from a pipe. And as far springs, spring rates back in the 1920s little tools, that tell an engine builder as my 30 year old memory serves me and ‘30s were negligible. This “attitude” what pushrod they need, while never on the research, aviation was also the continued on throughout the decades even taking into account the valve lift first use of a “roller tip” rocker arm, on afterwards. More importantly, and that will be used. Make no mistake; radial engines as far back as the 1930s, unfortunately, it bled over into the soon you cannot set rocker geometry without and perhaps before. In fact, to this to come roller rocker arm market, that knowing exactly how much the rocker day, I never cease to be amazed at the got its impetus in the 1950s. The first arm is going to move. foresight and creativity of both aviation person who made a working aluminum It seems logical that since a roller and automotive engineers in the 1920s, “roller tip” rocker arm for automotive cam can provide all the acceleration any and ‘30s, and ‘40s. Four valve Pent roof application belongs to my dear old and of today’s heads need, for any rocker combustion chambers, roller cams, fuel departed friend, the late Harland. Some geometry scenario, then why not set the injection, nitrous oxide, water injection, other garage efforts might have been rocker geometry to ONE STANDARD two stage superchargers, turbo chargers, getting tinkered with out in California that has the least amount of wasted and many other cool things we assume about the same time, but it is pretty well motion, and will always duplicate the were concepts of the last 20 or 30 years, undisputed that 1958 is the beginning of same percentage of cam information, were actually done and done quite well, what we know today as “the aluminum regardless of what cam you use? For seventy and eighty years ago. roller rocker.” Keep in mind that aviation understanding this, understanding a little Aside from the roller tip rockers roller rockers existed twenty or more history is always best. This ends a lot of of aviation long ago, the fundamental years before, but they were steel, they rhetoric. rules of rocker arm design were based were radial engines, and they didn’t Whenever pushrods leave their in- around the shoe tip, contact pad design comport to the automotive needle line paths to now have their end follow still used today. Many of you may know bearing aluminum body that Harland around the rocker shaft by any amount, that you can’t use a roller tappet on a introduced. this is LOST CAM INFORMATION. flat tappet cam, and of course vice verse Just like the flat tappet cam and The cam literally has to turn more not only because of hardness of material the roller tappet having entirely degrees to affect the same LIFT at a later difference, but because of “geometry.” different geometry because of where the point on the crank. Velocity, too, is lost. The principles of trailing motion and measurement for motion is made, the So you’ve lost duration, throughout the dynamics between something that is same rules apply to the shoe tip rocker entire lift cycle (not just overall), and making direct LINEAR contact upon versus the roller tip rocker arm. But when you’ve lost RATE of acceleration, by another object that is imposing or Harland made his silhouette, he didn’t slowing the rocker down. receiving a RADIAL (circular) is entirely allow for this, and inadvertently moved To put this in perspective, let’s take different than if that contact is occurring the axis of the roller roughly .300” of a simple even value of cam lift, like with a roller tip making or following the an inch higher than what it should have .400” to make a point. Rocker geometry contact. This isn’t rocket science either, been. The axis of the roller should have is usually thought about as only what and you can see how this happens by been in the same place as the contact is happening at the valve. In our .400”

engine professional APR-JUN 2010 21 ROCKER GEOMETRY JIM MILLER

cam lift example, and a 1.50:1 ratio a cam to gain as little as four or five him. It was a short list of real pioneers rocker arm, this would theoretically degrees if you think the engine needs it, to both cylinder head and valve train yield .600”. To have the optimum use but you just threw away more than that development back then, and Bill was on of in-line motion being converted into because of the PIVOT POINT on your the short-short list. But prior to Bill and circular motion, you need to divide these rocker arm didn’t establish the correct a few others like him of that era, rocker values into two equal parts. Engine angles with the pushrod and valve. geometry was totally ignored beyond the builders do this at the valve, but the real vague generalities of the 1/3 rule. But deal is happening with the CAM. The Why did this continue? technology in the cams and heads was cam is the source of the information. So Back in the early 1960s, because soon catching up. Right about this time, for the CAM side of this value, we’re there was so much inefficiency and in 1969/70, Chrysler approached Crane only ending up with .200”, which is one experimenting with cylinder heads, cams, Cams for a new camshaft for the factory half of our .400” cam lift. If you fail to induction systems and so on, this valve backed Hemi teams of Sox & Martin, place the axis of the tip of the pushrod train flaw went under the radar. Now I Herb McCandless and the “Motown at the proper length as to divide that was still a punk, barley sixteen in 1969, Missile” (later Mopar Missile). That cam lift accurately, so the rocker arm but if memory serves me right, it wasn’t development was the beginning of the is at a 90 degree angle, then you have until a real student of engineering and .700”-plus valve lift boundary being a pushrod that is going to move in and racing stuck his nose into the situation, broken. out more than it needs. The result will and started shifting the pivot point The late sixties and early seventies be wasted cam information that can around for his own purposes and seeing were really exciting times for factory require the crank to move several degrees distinct changes. He had some odd name muscle cars, and the stepping stones more to effect the same lift of the valve. car in a new class of drag racing, called of technology that has brought us to Those lost degrees were absorbed in the Pro Stock; I think it was “Grumpy’s where we are today. It all began back excessive motion the rocker arm had. Toy.” Bill Jenkins was one of the real in this limited, golden era. And the You will spend hours and hundreds of pioneers for many things in not just fixes fundamentals established then, cut in dollars to get a camshaft that is ground to problems, but also a more scientific stone, have not changed to this day to fourth decimal accuracy, and chosen Yunick-like approach to analyzing. either. They’ve only gotten repackaged, to give you a specific degrees of duration He didn’t follow other people; other renamed and resold, even though other at .050” tappet lift, and you will change people went out of their way to follow boundaries in valve lift, cylinder heads

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22 APR-JUN 2010 engine professional and so forth have been elevated. The Importance Rocker arms are a “radial” device principles for cam technology and WHY is rocker geometry so important? being ordered to do a “linear” thing. specifically rocker arm geometry that When you change the pivot point of They rotate on an axis, moving in a would soon come along in 1980, but where the rocker arm is, in relation to circle. But what they have to impart is spawned in 1973, have not changed to the valve tip, it CHANGES THE CAM. a straight line command. They get their this day. It doesn’t matter whose rockers you order from the camshaft, in the form use, it doesn’t matter what style rocker of IN-LINE information that they then Definition you have, it doesn’t even matter what have to ROTATE around an axis and application your engine is; whenever you then MULTIPLY it by some ratio, and What is rocker geometry? Rocker finally TRANSFER this result back to change location of the rocker pivot point geometry is “angles of motion.” It is not another IN-LINE component of greater in relation to the tip of the valve, you some linear reference point on the tip of movement. This movement has THREE are changing cams. You are changing all the valve, that trying to adjust the wear values: LIFT, DURATION (of lift), and three parameters simultaneously: LIFT, pattern will guarantee it being correct. VELOCITY (acceleration of lift). If the DURATION and VELOCITY (rate of What is correct? Correct, is “efficiency.” rocker arm does ANYTHING ELSE acceleration). It is having the least amount of wasted besides this, then it is NOT efficient, and The degree you change these depends motion being used to do the greatest SOME of this information is being lost. on how much you move the pivot point. amount of work (that is designed to Let me make a point about something And one or two of these three parameters on this. Your camshaft is ground to be done by the cam). This last point may be affected more than the other. is important, because the rocker arm ten-thousandths of an inch precision. It But if you don’t LOCK your geometry is computer designed to millionths of an can be used to add to the cam, besides in to the SAME THING all the time, inch, you (or your cam manufacturer) what it usually does by error, which is which has the least amount of wasted selected it for a division of duration take away from the cam. But I will get motion, then you are aiming at a moving values where you considered two or three into each of these below. I just wanted a target with every cam change. Whatever degrees important; any more was too big, simple “mission statement” that defines results you get from one cam to another and any less was too small. Hopefully by what geometry is and is not, so that the is tainted by the diluted effects of wasted now, you realize that moving this rocker following hopefully makes sense. motion in the rocker arm. pivot point will change this at the valve.

engine professional APR-JUN 2010 23 ROCKER GEOMETRY JIM MILLER

You just don’t know how much. Well, it 67 degree, because it references from motion itself. When using these tools, is MORE than the two or three degrees the deck), so you simply subtract the just like a dial indicator on the top of you think is important. In some cases 4 degrees that the tappet and pushrod a piston, as soon as the piston reaches it can exceed TEN degrees, and is often “aim” toward an already inclined 23 perfect TDC, you will have two or three five or six degrees. As if this isn’t enough degree valve, and you end up with simple degrees of crank movement before you reason, consider this: It is that value of math: 19 degrees. Every engine is unique. see the dial indicator move. There is a loss throughout most of the lift cycle, not The SB Ford is 20 degrees for this same float time there, and so too is the effect just total – where you’re only inclined value. This is what rocker geometry by using a tool on the roller tip of a to measure it from, and where your cam needs to be designed to, otherwise your rocker to measure in and out motion. It card is limited to. That is what’s missing. efforts of installing the rocker arm floats enough to allow the valve lift to Engine builders only check at FULL accurately will be limited to just one side be off by .005” to .010” or more. But if LIFT. They check rocker ratio and total of the rocker or the other. you can set it dead nuts within .002” to valve lift, and that’s that. But when your .003” without having to buy such a tool, rocker geometry is off, you’ve lost those What Rocker Geometry IS NOT why wouldn’t you do it in a more precise degrees of duration throughout most Rocker geometry is NOT where the way? of the entire cam profile. Which means roller (or contact point) is at on the rate of acceleration is lost, but you may top of the valve. Forget that. Rocker Alternative Geometries only see a small change in lost valve lift, geometry, especially is NOT the idea Before getting too deep into philosophies, thinking the difference is just flex in the that you want to place the roller or wear history and facts, let me restate the key rocker ratio. pattern (shoe tip rocker) in the “middle point of what rocker geometry is, then I of the valve.” Forget that, too. will mention the comparative arguments Two Geometries The valve tip is everything. It is people (and companies) have made Rocker Geometry is the correct DESIGN ground zero. This is where all leverage, against this. MID-LIFT geometry is and INSTALLATION of the rocker arm and the full stroke of valve lift begins, rocker geometry that has the ultimate so that its relationship to both sides of this is our reference point. There are “efficiency,” in that it is doing the up-and-down motion is fully realized by many ways to measure rocker geometry, greatest amount of work with the least BOTH. This is of course, the pushrod, but there is ONLY ONE way to SET IT. amount of effort. It has the least amount and the valve (respectively). Now, you can set rocker geometry in of wasted motion in the pushrod and The rocker arm is a RADIAL tool, the closed position, or you can set it in valve, commonly referred to as the “in- asked to do a LINEAR job. It pivots the MID-LIFT position (half open), or and-out” motion. It affects the maximum around an axis in a reciprocating you can do like most people have done, response through linkage of whatever the radial (circular) motion, and has all the and simply roll the engine over a couple cam’s instructions are. If you don’t have dynamics that anything revolving around dozen times watching the valve open geometry set precisely, your consequences an axis will have. But on either end of the and close to see your witness mark (foot range from simply losing a little cam rocker arm’s connecting points are two print) atop the valve tip and play hit information at the valve, to excessive side other instruments each live and breathe and miss with chasing a “minimal wear loads in various directions, on various by the laws of linear (in-line) motion. pattern.” parts that will at the very least rob you Now all this may sound like “rocker The problem with this latter point is of power. In more extreme cases, wearing arms 101” and we may all know this, that this is a symptom, it is not geometry. out parts or outright catastrophic part but few people I have found over 35 Granted, when you have the rocker failure may occur. years, seem to understand how sensitive, geometry set properly, you will have the There are arguments to using precise and important this observation least amount of wear pattern, but to different geometry than mid-lift. I simply is. I think this true because most treat try to set geometry through moving the don’t agree with them because they both the design of the rocker arm and its rocker and trying to see how small you violate the principles of efficiency. One installation with casual regard. can get it, is better than nothing – not of these theories is to adjust the rocker To have the “most efficient” design quite good enough. You can easily be off arm’s height so it reaches a 90 degree and use of the rocker arm requires TWO by .005” to .010” (or a great deal more) relationship when the valve is about 2/3 things: (1) The rocker arm must be on even seeing this actual width, let alone open, not half open. The logic being, that designed to mirror the inherent angles of measuring it. And being off .010” on the the spring loads on the rocker body are each engine’s pushrod to valve geometry. horizontal plane of what you are really less. Another reason I’ve heard is that it (2) Rocker geometry must have an trying to measure, which is the vertical accelerates the valve in the “mid-range” accurate location of its rotational axis plane (valve lift), will multiply out quite better, thus making more power. Other with the valve tip’s height. The first easily to .030” or .050” or .080” or variations of this approach shift the of these is called DESIGN geometry; more in your error of where the trunnion rocker arm’s pivot point higher on the and the second is called INSTALLED is to the valve tip! Those kinds of errors valve tip to create this 90 degree effect geometry. will cost you several degrees of crank sooner in accelerating the opening of Every engine has an inherent acute rotation to open the valve a like amount. the valve at a lower point of lift, thus angle (I call the “attack angle”) which What about using a tool, or dial increasing what is termed “area-under- the pushrod leans either into or away indicator designed to measure this in the-curve.” from the valve centerline. The small and out motion, resting on the spring Both of these are a way to add block Chevy for instance is 19 degrees retainer? Well, this is a better way of different cam information to the valve positive (leans into). This comes from the same thing, but it is still measuring by using the rocker arm’s geometry. The the engine block having a 4 degree a horizontal plane for a vertical plane only reason you would use the rocker angle of its tappet bores with the piston result. Error can be off several times arm for creating a “second dynamic” cylinder centerline. The head is already more than measuring directly in the of valve acceleration, is if the cam was a 23 degree valve angle (actually it is a vertical plane, or parallel to the valve unable to give you the acceleration

24 APR-JUN 2010 engine professional ROCKER GEOMETRY JIM MILLER

you needed. Now, in some cases this full lift. Because it will always mirror BOTH sides of the rocker, because you limitation exists. They would be flat whatever its settings are. didn’t install it accurately, you lose twice! tappet cams, of mechanical or hydraulic To see the difference all you have Only 90% may go into the rocker from operation, and an engine where cylinder to do is take an old fashioned needle the cam, and of that, only 90% comes heads had the flow potential, and/or pointer torque wrench and turn the out, or 81% goes to the valve. I’m of cubic inches had the demand which engine over without spark plugs and course rounding things off for example, required a crazy acceleration to mid- measure the drag. Then set the geometry but the principle is the same. lift flow values off the seat. In other to MID-LIFT, and see the difference. I should add one other point here. words, the engine was so big, and the I’ve had people do this and fall out of Everything is “net.” So if you have heads were so big but for rules or some their seats. They see as much as 45 foot those cute little “checker springs” laying other illogical reason, the cam they pounds or more and LESS torque to around, find some other use for them, HAD TO USE was a flat tappet profile turn the engine over. Usually it is 15 or because outside of holding a valve in a that had limited “rate of acceleration” 20 foot pounds but it depends on how head for display where someone can use by its limited tappet diameter and base much spring pressure you have, how their finger pressure to push the valve circle constraints. I won’t get into cam complex the rocker geometry is (Hemi open, they have little use. You need the technology and limits, but that is the first versus SB Chevy, for instance), and how REAL running springs for any geometry reason I can think of for using the rocker wrong their geometry really was. Either setup. The same goes for checking flex or arm as its own cam tuner. In some Stock way, it quickly shows you the definition piston to valve clearance or anything else classes where the original cam must be of efficiency. If they MAP their valve critical. Checking springs ADD about used, a creative (and well funded) engine acceleration, throughout the entire lift .040” (or more) NET valve lift to your builder can play games with rocker curve, then I need say no more. The best engine. Or, another way to say it is you geometry to change valve lift rates, but way is to DIVIDE the arcs EQUALLY will LOSE .040” or more NET valve lift these are very limited differences, usually and standardize this for all cam and when you put the heads together with the not worth the trouble, and most of all, in cylinder head testing. Change your cam running springs, compared to whatever ALL these examples, there is going to be as you need. you measured using the checking springs. detriments that outweigh benefits. This is true across the board, flat tappet In the first place, for those who Setting Geometry cams, roller cams, aluminum rockers, have an engine of large flowing heads, As mentioned, the rocker geometry has steel rockers – it makes no appreciable and big cubic inches, or heads for very TWO considerations: The accuracy of difference. high rpm’s, they will have the benefit how it is installed, which I have always Later in this article, I will offer of using a roller cam. So the issue of referred to as the “installed geometry.” installation and assembly tips but for how fast you can open the valve is not And second, how accurately it is now, here is the easiest of accurate ways even a consideration, because by nature designed, which I have always referred to set INSTALLED rocker geometry: of roller tappet geometry, any value of to as the “design geometry.” This second The closed valve position is the easiest acceleration up to and through suicidal item relates to where the adjusting screw and the best. The cam must be in the parts destruction can be implemented or cup is placed in the body, and at what closed position, on its base circle. Heads on the cam profile. And for those classes angle. are assembled to the engine, with no where a flat tappet cam is required, the As an engine builder, you can move pushrods in place. You must know what cubic inches and head limitations of most the rocker arm up and down to the valve your “net” valve lift is supposed to be rules I’ve seen over the years, fall within tip in setting that side of the rocker’s (we can get nitty-gritty later). You will airflow and rpm limits that a flat tappet effect, but you can’t do too much about subtract any valve lash so you have an cam fit just fine. Too many times, cam the pushrod side, and that is where the accurate “net” lift. companies talk customers into roller information comes from. That is up to For stud rockers, put it in place with profiles that are not needed, and in fact the rocker arm manufacturer. an adjustable pushrod. You don’t need don’t make as much power as a well Of the two sides, however, the lesser the poly locks; just let it set loosely on chosen flat tappet would, because it takes of evils to shoot for is setting the VALVE the stud. Knowing your net valve lift, more power to operate the roller. Using side (or installed geometry) as closely DIVIDE it in half, and write it down rocker geometry as a second cam shaft as possible, because (a) this is where on a piece of paper. Then, lengthen or is not a good idea. The velocity of the the motion is constrained by the valve shorten the adjustable pushrod to raise rocker arm increases where its motion guide, (b) this is the side that has the and lower the back of the rocker until line reaches a 90 degree angle, and trying greatest motion, (c) this is the side that you get the center of the trunnion exactly to pick a particular segment of the valve has the valve spring, where harmonics HALF of your net valve lift BELOW lift that you want to impose that thinking are generated and amplified, (d) and this the center of the ROLLER TIP. If for over what the cam manufacturer has is the side where all of the foregoing example, you have .600” net valve lift, done, is bad news. But there’s another multiply into a measurable resistance then this would be .300”. Keep in mind point to consider on this issue. value that generates heat, robs power and that I refer to “center” of the trunnion The rocker arm is a symmetrical creates additional friction. The pushrod and roller pin. It is their axis that is what device to whatever geometry it is set at. is, by comparison, free floating with the you are measuring. Some are easy to see In other words, whatever acceleration it rocker body’s movement in and out as and some are not. For those with flat exhibits on the opening side of the valve well as up and down, and it is moving machined surfaces, take a scribe, measure gets reversed on the closing side. Simply, less (the cam’s lift). But make no mistake and mark these centers as best you can. if you set geometry with a HIGH pivot about it, when the pushrod side is not But the main trick is that you want to be point, so it increases its velocity quickly performing to mid-lift geometry, it is sure you measure this from a precise 90 off the valve seat, then slows to full lift... losing information. The upside though, degree reference to the valve centerline. Guess what? It’s going to accelerate is that whatever is left, is getting through To accomplish this, you are best served back to the close position when it leaves to the valve. When the geometry is off on to use the top of the valve spring retainer.

26 APR-JUN 2010 engine professional Simply lay a short machinist ruler (or or have a stand that has a surplus of you’re degreeing your cam, but you’re something comparable) atop the valve metal that you are machining exactly measuring motion at the valve. Only spring retainer, and pass it along the side what you need away. But you can take a instead of just picking up points of lift of the rocker arm. measurement of the stand height without to compare to the crank, as you would When you have the height installed using a rocker arm. Just bolt the stand with the .050” tappet lift measurements accurately, the trunnion will be exactly down to the head with a couple of bolts, on a cam card, you will be creating a HALF of your net valve lift, below the lay the shaft in the stand’s bed-way, and graph all the way through the entire roller tip centerline when the valve is use a machinist square along the side cycle of valve lift, opening and closing. CLOSED. As it opens, and moves to of the valve (or spring) and shoot the If you are fortunate to have a CAM exactly mid-lift, the axis of the roller long end along the top of the shaft, so PRO or CAM DOCTOR, or something tip will have dropped down to be there is a gap beneath it and the top of similar, life is good. If not, you can do straight across from the trunnion and an the valve. Everything is about ﬁnding the old fashioned way. You will need imaginary line that runs between them (I the centerlines, and being creative about graph paper that can be found at art call the motion line) will form a precise doing this, while being accurate at the stores, engineering supplies and many right angle with the valve centerline. The same time and measuring at an accurate science or school supply providers. You roller is at its farthest point across the right angle with the valve. need a dial indicator that you’ll mount valve when this happens. When the valve It makes no difference where the directly above the valve spring retainer, continues to open the second half of its wear pattern is at on the top of the valve, nearly fully compressed so you follow lift, to full lift, the roller will have moved when you have correct mid-lift geometry, the valve’s stroke fully – and you want exactly an equal amount BELOW the and providing the pattern is “on top of to be sure the indicator stem is lined up trunnion as it was above the trunnion the valve.” (Running off the edge of the parallel with the valve. As with setting when it was closed. And the roller will valve is not good.) your cam, you need to have a degree be at its closest inside point on top of the wheel in place on the crank and zeroed valve. You will also have the ultimate Graphing the Cam at the Valve accurately to TDC of the piston. least amount of roll across the valve. To see what is really happening at the With the above in place and ready, For shaft mount rockers, it’s a little valve, you need to check your rocker’s you have TWO choices to how you different, but the principle is the same. motion by measuring it at the valve. The measure this; which are merely opposite With shaft rockers you must use shims, best way is to essentially treat this like perspectives. You can choose an even

engine professional APR-JUN 2010 27 ROCKER GEOMETRY JIM MILLER

number of crank degrees you move and trunnion of a needle bearing roller position. But as long as you stay focused through to measure valve lift, or you tip rocker arm. Even finding the axis is on what you need to do, you won’t try can choose an even number of valve lift not easy. Setting it by the same rules is juggling things around outside of the real to measure degrees of crank rotation. best simply for standardizing one cam to priorities, based on a false idea of what It doesn’t really matter which you use, another. Only your reference is the actual YOUR rocker ratio ought to be. It can because it is the comparison against contact point of the pad itself. When it be all over the place from .05 less, up to other like tests that is important, and is at mid-lift, you will have a 90 degree maybe close to what it is supposed to both need to be the same. You don’t have relationship between the TIP of the valve be, simply because many manufacturers to be too crazy about fine increments and the center of rotation for the rocker began in the closed position, and then here, just choose valve lift jumps of arm. But finding that center is tricky, started moving specs around from a hit maybe .020” and write down the crank because these are usually ball fulcrum and miss until it got close. Once it did, movement; or choose 5 or 10 degree rockers, and they are surrounded by the they left it there. Over the years, more crank measurements and write down the stamped metal that has no clear axis to consistent manufacturers with the least valve lift. it. One solution is to put a stud upside amount of broken parts have been the Personally, I like the second method down in a vice and rotate it carefully of using a fixed crank stepping, and then while observing the fixed point on its side model other newer companies would noting the valve lift. This goes directly that most closely represents where the copy. But the mistakes get copied, too. to the points I make about “area-under- center is, then making a little felt tip pen You have to always check things for the-curve” that you are looking for. mark. This would then be set exactly half yourself, forget the advertising. If you For those new to this term, area-under- of your net valve lift below the valve tip do, then you can’t blame anyone but the-curve refers to the VALVE LIFT in the closed position. It’s not as accurate yourself. CURVE as charted across a graph of as fixed points to set calipers against, time (meaning crank degrees), and what but it will get you very close if you have Installation, Measurement and most engine builders agree, is that lifting patience and a sharp eye; and with shoe Adjustment Tips the valve as quickly as possible and as tip rockers, the amount of error you soon as possible, while setting it down might be off will have no measurable Figure 1 quickly after it has hung open for as effect in cam efficiency as it would with Here’s the typical stud mounted needle broad a period of time as needed, but needle bearing roller tip rockers. bearing roller tip rocker shown in its without being too fast to damage the CLOSED position. Dimensions are not valve train from excessive “bounce” is Twisted Rockers shown, because they are relative to your what everyone wants. So, when you want Unfortunately, engine builders are led to see the gains and losses in this area into a false security by stud mount installation, which is relative to your from inaccurate rocker geometry, you’re rockers sold for heads that shouldn’t use NET valve lift (after lash, if used). The really looking for wasted time when the them. These are aftermarket heads with important dimension you want to find crank is moving more than it needs to pushrod offsets that require an offset and set, to be HALF your net valve lift the valve the same amount. So, if pushrod cup, or adjusting screw. Shaft lift, is the ROCKER HEIGHT, shown you standardize your testing to the same systems usually have this adequately to the upper right, and illustrating the valve lift measurements, the gains and fixed, but when heads are sold with studs cumulative value between the ROLLER losses in the crank are readily seen. in them, that clearly need to be removed axis and TRUNNION axis. The other Once you’ve charted one rocker for a shaft system, this is the false sense references are shown with regard to their geometry setup, perhaps the one you’ve of security one gets. The two rockers heights above and below (respectively) been using, now make the changes with shown here is exactly what you DON’T the valve spring RETAINER. All pushrod length and/or stand height (for want to have (Figure 4). measurements for where the trunnion shaft systems) to meet with what I hope When you have this much rotation, is sitting, is referenced to the top of the I have informed about earlier in this the roller tip does NOT lay flat on the retainer, marked here by TRUNNION article. You will often see that PEAK valve tip, and as it opens the valve its REFERENCE. Raising and lowering your valve lift is very close to the same, but energy is making a cross sword slash rocker’s tail by adjusting the Pushrod as much less at other points in the curve. across the valve tip that rounds off the needed, will set this. Although shown That is the lost information. The degree top of the valve tips, side loads the guide in this illustration, the adjuster doesn’t of this will depend on many factors on the X axis (length of head), and shifts even need to be here, as it will only get in that take another story to itemize. But side loads to the bearings in a way that the way. Let the rocker sit loosely on the the bottom line is; you will appreciate often push or break one prematurely. It’s study, with its adjustable pushrod and set how important it is keeping the same bad news, costs horsepower and breaks this trunnion reference as needed to get geometry, for making sure your cam parts. the trunnion exactly HALF of your NET changes show results that are directly accredited to them and only them. Ratio & Geometry valve LIFT. Otherwise, your information is tainted. If the rocker geometry is off, then so If using a HYDRAULIC TAPPET, too is the ratio. There’s some good news be sure that it is fully extended during Shoe Tip Rocker Geometry though: Don’t worry about it, because this check. You can prime the motor As with aluminum rocker arms, there are very few of the rocker arm makers did. to do this. After getting your pushrod different design geometry shoe tip rocker The history of rocker design didn’t have length, ADD .020” to allow for hydraulic arms, but the priority for adjusting the much accuracy involved. There was no tappet compression during actual engine valve tip side is still there for the same standard, because there was no need for operation. Order EXACTLY what you reasons. However, you don’t easily have such accuracy in the old days. Rockers need for pushrod length, rounding off to the same accuracy as you do with picking always were, and to a great degree, the nearest ten-hundredth of an inch (two up precise center points on the roller still are designed in the closed valve decimals).

28 APR-JUN 2010 engine professional 3

Figure 2 actual roller on his outline to calculate be to leave the STUDS on the same This is the references to pay attention this. Setting the roller’s diameter in direct centerline as the VALVES, and force stud to for setting a SHOE tip rocker arm position where the scuffing surface was rocker manufacturers to put their offset for your NET valve lift. As mentioned, at, instead of the roller’s centerline is for the pushrod solely in the rocker arm, finding the centerline of the fulcrum’s what is wrong here. It was the beginning with an offset cup. This is what Ford rotation is required first, since this of a duplicated error that would last over cutaway only shows where the axis a quarter century. did on their N-Head, and it is the best would be. This is not easily seen on the way. Otherwise, you need a stand (shaft) outside of a stamped or cast rocker body Figure 4 system. Here’s the bottom line: You can so, you need to simulate this rotation and As explained in text, this is the real never have an inline valve array cylinder make a mark to reference to. example of what you don’t want to do. head, like a SB Chevy, Ford, Mopar, etc., But it is very typical, and the problem is and NOT have the rotating axis of the Figure 3 propagated more by the head companies rocker’s trunnion be IN LINE with the As crude as this may look, this is luring engine builders into a false sense of CAM. Any twist at all, is COMPOUND an actual recreation of the drawing “acceptability” to such an installation, by illustrated to author by the late Harland selling their heads with studs in them that geometry, and will make it impossible Sharp, explaining his original layout of a have this stretched over placement, when for the roller to lay flat on the valve, or rocker silhouette on paper before scribing they’ve had to move the pushrod for follow the correct straight down path on where the roller would be, and using an wider ports. The better alternative would the Y-axis.

engine professional APR-JUN 2010 29 ROCKER GEOMETRY JIM MILLER

Figure 5 You MUST know this for your engine. lowered to bring the centerline of the This represents the STAND (SHAFT) It is impossible to set correct mid-lift shaft, half of the net valve lift below rocker system, with only the most rocker geometry without knowing the center of the roller). Usually, with important things to be considered; your net valve lift. The ROLLERS are shimming, you might have ﬁgures namely, the STAND, the SHAFT and the shown here in their two critical states, that make you ADD shims to get the ROLLER TIP. the dashed version representing where correct value. The valve is shown for angle and the roller will be when it has opened the location, and is shown here in the valve to FULL lift. But the valve is only As you might notice, we are using the MID-LIFT point of motion, as noted shown in the closed position, as is the outside diameters of the shaft and roller to the side. THIS IS THE GOAL. The solid roller atop it. to calculate this standard from, because roller and shaft are to be horizontally First, take HALF of your ROLLER these are easily measured with common in line with each other as measured to a diameter; and HALF of your SHAFT tools. But it is their centerlines that are so perpendicular right angle with the valve. diameter, and ADD them together, important. you will come up with a “standard.” Figure 6 In this example, that standard would This shows a SQUARE (crosshatched) be .521”. This comes from a roller being used to lie against the valve and diameter of .480” and a shaft diameter atop the shaft of your stand system. of .562”. Why half? Because this ﬁnds The stand is bolted to the head, and a our CENTERLINE for each. It is always shaft is laid in position to now make the centerline that you are setting with this check while the valve is in the rocker geometry. CLOSED position. The cool thing about Second, write down the height of the this is it can be done on a work bench. SHAFT’S TOP above the valve tip. No springs, no anything, just the parts Third, you need to write down your Jim Miller has been involved in the racing industry for shown. If you are doing this with an MID-LIFT value (half net valve lift). assembled head, you can run the square more than 30 years. Jim’s roots stem from amateur racing, starting at 16 but by age 17, he was the along the side of the valve springs, Here’s the TRICK: youngest Ford trained line mechanic authorized to do providing they’re uniform diameter. With these three things written down: Otherwise, you may have to use the valve warranty work on all of Ford’s factory muscle cars. spring retainer technique from our stud 1. Subtract HALF net valve lift from At the age of 21 Jim was invited to take over Chief mount example. your STANDARD (.325” from .521” Mechanic duties for Dyno Don Nicholson’s Pro Stock in this case, for a sum of .196”). Maverick, which Jim respectfully would decline – Figure 7 2. Subtract the sum of the above (.196” passing the opportunity on to Jon Kaase. Jim holds Here’s the stand (shaft) mount system in our example) from your rocker’s a number of patents for valve train design and is the shown in the close position, and our height (for our example this would proprietor of MID-LIFT Precision Geometry. Jim can be example here has a NET valve lift of be .350” minus .196” = .154”. This reached at 1777 Blount Rd., 501 Pompano Beach, FL .650”, or a MID-LIFT value of .325”. is how much the SHAFT needs to be 33069. Phone 954-978-7001.

30 APR-JUN 2010 engine professional