Piston & Rod Orientation Including a discussion of major and minor thrust

BY MIKE MAVRIGIAN

When it comes time to install pistons and isn’t an issue of the bearings themselves ROD BEARING TANGS connecting rods, orientation of the two but merely serve to provide a reference for The grooves in the rod big end and components relative to each other and rod orientation. cap and the protruding tangs on the relative to the block can sometimes lead If there is no noticeable chamfer on rod bearings exist to facilitate bearing new builders to wonder about direction. either side of the rod big end, the bearing installation and are not specifically This brief article should help. placement on the side of the rod that faces intended to prevent bearing “spin.” The the fillet should be slightly spaced away bearing crush that is generated when WHAT DIRECTION DO THE RODS GET from the fillet to prevent the bearing from the cap is properly tightened prevents INSTALLED TO THE PISTONS? digging into the fillet radius. bearing movement. The grooves in the If the big end of the connecting rod rod and cap and the tangs on the bearings features a larger chamfer on one side, SQUIRT HOLES merely serve as an installation aid in order this side must be installed facing the to align the bearings during assembly If an oil squirt hole is featured in one crankshaft’s journal radius fillet. If the (locating upper and lower bearings side of the rod’s big end, this oil hole is rods are designed for use on a crank correctly fore/aft). Bearings as-installed intended to allow oil to be squirted to that does not feature a radiused fillet, feature the ends slightly protruding the rods may not feature a large chamfer opposing pistons, so the oil squirt hole beyond the parting line. The bearings are on one side. Rod orientation can then in the big ends should be oriented so locked in position and secured as a result be obtained by orienting rod with its that they face the cam. These big end of radial bearing crush when the cap bolts bearing tang grooves biased toward the squirt holes are not intended to provide are fully tightened to specification. pan rail (outside) or cam side (inside). For camshaft lubrication as some folks have OE engine designs have begin to use example, SBC and BBC bearings tangs are been led to believe. An oil hole in the rod tang-less bearings (Chrysler 3.7L and positioned toward the outside (toward the small end (whether on top or at an angle) 4.7L engines are an example). Eliminating pan rails). Other designs may specify that is to simply allow splash-oil delivery to machining of grooves in the rod and cap the tangs are located to the inside. This the wrist pin. and eliminating the tang on the bearings

Examples of two opposing-bank asymmetric pistons. Each piston features a wider skirt for the major thrust side and a narrower skirt for the minor thrust side.

50 OCT-DEC 2016 engine professional Another example of an asymmetric piston. Notice that the pin bosses are closer together to accommodate a An oil squirt shorter (and lighter) wrist hole at the pin. While it’s difficult to see rod small by eye, the pin centerline is end serves biased 0.020” closer toward to provide the major thrust side in lubrication to order to tune the pin’s pivot the wrist pin. point balance in order to compensate for the difference in major and minor skirt size/ mass.

A view of a small end squirt hole at the exterior of the small end.

A larger chamfered oil hole at the top of the small end on some rod designs provides a “funnel” for improved oil reservoir for floating pin lubrication.

When a piston maker specifies a precise skirt height location for diameter measurement, this represents the area of the skirt “barrel” profile that will experience the highest cylinder wall loading.

Asymmetric pistons may also feature an arrow that indicates piston orientation The underside of this piston features an “L” suffix, indicating that this piston is relative to the front of the engine. This provides a handy visual aid in installing intended for the left bank of a V engine. Pistons intended for the right bank will the pistons so that the major thrust side skirt faces the major thrust load feature an “R” suffix. within the cylinder bore.

www.engineprofessional.com 51 PISTON & ROD ORIENTATION BY MIKE MAVRIGIAN reduces manufacturing cost. Again, the left bank and the intake side of the of the left (driver) side cylinders. The the locking tangs only exist to provide right bank. If the pistons are symmetric minor thrust side experiences force on the assembly alignment. When installing and do not feature an offset pin location, compression stroke. no-tang bearings, they must be centered piston orientation won’t matter, as long This difference in force at each side on the rod and cap bearing bore surfaces. as the valve pockets (if any) are located of the piston is caused in part by the We’ll be seeing more and more of these appropriate to the valve locations. operating angles of the connecting rod applications from the OEM side. during its travel. SKIRTS AND MAJOR/MINOR THRUST During the firing cycle, the load ROD OFFSET The shape, area of mass and weight of experienced on the major thrust side skirt Offset designed into a rod places either a piston’s skirts play a major role in can be as much as 10 times greater than the large or small end centerline slightly managing friction and in stabilizing the the load experienced on the minor thrust offset from the centerline of the rod piston during TDC and BDC transitions. side skirt. The difference in skirt loading beam. This is to accommodate engine Here we’ll discuss the role of the major will vary depending on variables such as designs where the centerline of the and minor thrust sides of a piston and the crankshaft stroke, connecting rod length cylinder bore is slightly offset from the development of asymmetric skirt designs and peak cylinder pressures. fore/aft intended location of the rod intended to minimize weight while Since asymmetric pistons are bank- bearing radial centerline. Depending on maximizing efficiency. specific, each piston will be labeled for the specific engine deign, the rod may Piston skirts are not perfectly round, right or left bank position. The dome feature an offset of as much as 0.100” or and each side of the piston experiences may also feature a laser-etched arrow more. different levels of loading, relative to that indicates piston orientation toward If in doubt, during test fitting, verify the intake and exhaust sides of the the front of the engine. that the small end of the rod is centered cylinders. Skirt design plays a major role on the wrist pin between the pin bosses. in accommodating these forces in terms Major thrust side of durability and performance, as well as When the piston is pushed down during DO REVERSE ROTATION ENGINES GET piston weight. the power stroke, it experiences resistance PISTONS INSTALLED BACKWARDS? The piston skirt area is slightly as it attempts to turn the crankshaft. As On a reverse-rotation engine (where the “barrel” shaped to provide an adequate load increases, the amount of resistance crank rotates counterclockwise when surface load against the cylinder wall increases. During this resistance, the viewed from the front of the engine), while reducing friction. The amount rods are installed similar to a clockwise of surface area must accommodate the rotation engine, where the larger load, while providing piston stability to chamfer side of the big end faces the minimize rocking relative to the pin axis fillet. However, if the pistons feature an as the piston moves down from TDC and offset pin, the piston must be installed back up from BDC. “backwards” relative to installation The piston experiences a “major” in a clockwise engine. The pin offset and “minor” thrust force at opposing is biased toward the major thrust side sides of the piston skirts. The major of the piston. In a clockwise-rotating thrust face is the side of the piston that engine, the major thrust side is at the receives the thrust on the power stroke. intake side on the left (driver) bank and As viewed facing the front of the engine, the exhaust side on the right (passenger) if the crankshaft is rotating clockwise, the bank. In a reverse-rotation engine, the major thrust face is on the left side of the thrust sides are opposite: the major thrust cylinder (the exhaust sides of the right/ side will now be at the exhaust side of passenger cylinders; and the intake sides

A view of the major thrust side. Note the width of the skirt. By contrast, this is a view of the minor thrust side of the same piston. (Courtesy JE) Note the reduced width of the minor-side skirt. (Courtesy JE)

52 OCT-DEC 2016 engine professional If a large chamfer is present as shown here, this side of the rod faces A rod big the radius end’s non- fillet of the chamfered crank journal. side faces the adjacent rod big end.

piston side load is forced to one side (the major thrust side), which places more force (with subsequently increased friction and Bearing tangs register potential wear) on the thrust side of the to the tang grooves in cylinder wall. If the piston dome features the rod and cap simply a reference dot or other mark, it’s critical to provide a locating aid to install the piston with this mark facing to place the bearings the appropriate direction (usually the mark in the correct fore-aft position, to eliminate indicates the side of the piston that should potential mistakes in face forward). The piston side loads on the bearing location. Bearing major side tend to increase with the use crush that results from of longer stroke and with forced/boosted cap tightening serves induction pressures. Again, assuming a to lock the bearings in clockwise-rotating crankshaft, the major place to prevent bearing thrust side will be at the exhaust side of the spin. Note how this bearing is registered engine’s right bank and the intake side of slightly biased away the left bank. from the chamfered side to prevent the bearing Minor thrust side from digging into the The piston’s minor thrust side is directly crank fillet. opposite of the major thrust side. The minor thrust side is forced to the opposite side of the cylinder wall as it moves up on the compression stroke, due to the resistance generated by meeting the air/fuel mixture. The role of the minor thrust side is basically to provide piston stability, with the major thrust side taking the brunt of the cylinder wall contact. Due to its “less force” role, the minor thrust side skirt can be narrower, saving weight, without sacrificing strength. In order to address, or “fine tune” these forces between the major and minor thrust sides, asymmetric pistons have been developed that feature two different-size skirts. This style of pistons is specifically designed with a larger (wider) skirt on the thrust side and a smaller skirt on the minor thrust side. This provides a greater “footprint” for the major thrust side, Bearing crush occurs due to the radial pressure exerted on the bearing as the where it’s needed the most to handle a cap bolts are tightened. (Courtesy MAHLE Clevite) higher degree of thrust loading, and allows

www.engineprofessional.com 53 PISTON & ROD ORIENTATION BY MIKE MAVRIGIAN

FEA takes a snap shot of the piston’s stress levels This FEA (finite element analysis) view shows at the worst case scenario and differ greatly even stress forces at both major and minor thrust from an engine that’s at part throttle. The image The stress of the piston under major stress. sides (note the dark dome areas), even though represents a stress level plot. The high stress (Courtesy JE) skirt areas differ in area. The offset location of areas are shown in red in accordance with the the wrist pin aids in balancing-out the pivot point. chart on the right. This is a simulation of stress (Courtesy JE) under firing. (Courtesy JE) the piston weight to be slightly reduced Another benefit to the asymmetric pins. According to JE, a typical weight by featuring a small footprint on the approach with regard to skirt mass and savings is in the 10 gram range. opposite/minor thrust side where the profile is increased piston ring sealing NOTE: The contact pressure FEA force is less. During the power stroke, and ring stability. Basically, the dedicated images show contact pressure specifically when the piston changes direction at top major and minor thrust skirt design between the skirt panel and the bore. It’s important to analyze both skirt profiles dead center, combustion pressure pushes coupled with a slightly offset wrist pin on an asymmetrical piston design even the piston down, and at the same time directly addresses ring performance in though the minor thrust experiences a pushes the thrust side of the skirt towards addition to reduced wall friction. the cylinder wall. much lower amount of pressure. On a Citing JE Pistons’ “asymmetric” symmetrical design, typically only the Offset pin major thrust is analyzed. Stress images design as an example, in their FSR Asymmetric pistons will also feature an show how the stress at the skirts affects (forged side relief) line, a wider skirt area offset wrist pin, with the pin centerline the rest of the piston. is featured on the major thrust side, and biased from zero towards the major Hopefully, this article on piston and the pin bosses are relieved at the outboard thrust side by 0.020”. This slight rod orientation helps you. Remember, sides to allow the use of a shorter (and it’s best to mark rods before disassembly. lighter) wrist pin. offset tends to balance the piston to accommodate the difference in skirt mass AERA’s tech team often receives The asymmetric piston design concept questions regarding this topic and they and to compensate for and alter the effect was initially developed years ago for asked me to write an article to better of rod angle, transferring a bit of force specific racing applications. While it explain. Answering questions and helping didn’t get much OE backing then, there is away from the major thrust side. with technical issues regarding engines renewed interest in it now and its use has Again citing JE’s development in this is AERA’s main mission. Toll-free access trickled down to street applications, with area, the asymmetric design allows the to four full-time tech experts is just the GM LS platform as a good example. use of shorter, stiffer and lighter wrist one of the many benefits of an AERA membership. To learn more about AERA, visit www.aera.org and join An anti-friction coating today. You’ll be glad you did and most on the skirts aids in oil likely thank me tomorrow!n retention and provides additional friction reduction especially during cold starts. While this can more prominently benefit the major thrust skirt, applying the coating to both major and minor skirts provides a Mike Mavrigian has written thousands of technical lubricity “back-up.” articles for a variety of automotive publications and many books for CarTech and HP Books. Contact him at Birchwood Automotive Group, Creston, OH. Call (330) 435-6347, email: [email protected] or go to birchwoodautomotive.com.

54 OCT-DEC 2016 engine professional