1.3mm/0.050in wide and a normal three angle valve seat is very suitable (45 degree seat/30 degree top cut/60 degree inner cut). The inner cut blends the 45 degree valve seat into the port throat and the top cut blends the 45 degree valve seat into the combustion chamber. The whole exhaust port passage- way must be enlarged throughout. The valve throat is ground or bored out to 34.5mm/1.358in and the original valve seat machined to suit the new valve size. The port outlet is taken out to the size of the exhaust manifold gasket aperture (oversize - lmm more height and width - exhaust manifold gaskets are available). The passage- way is ground out quite extensively, but note that while the floor of the port should be cleaned up it should not have any serious amount of material The areas of the combustion chamber to be reworked are shown here (see text for key). removed from it, especially where the The amount of material that is removed does vary depending on the proximity of the port runs into the valve throat area. gasket and bore wall. The sides of the exhaust ports and the to the full size of the standard gasket of gasket is used). This modification roof certainly have plenty of material using the gasket as a template. Over- gives a considerable increase in which can be removed and, once sized gaskets which allow an even exhaust port area because the stand- modified, bear little resemblance to the larger port exit cross-sectional area are ard 'as-cast' dimensions at this same originals. available, but are only required for point are 32.3mm/1.275in in height by competition engines which will fre- 24.0mm/0.950in wide. Material is Enlarging exhaust ports quently run to 7800rpm or more and, taken equally off the sides and roof (large valves) as a consequence, have large primary (nearest the camshaft) of the port but The modifications described here exhaust pipes (47.62mm/1.875in). not from the floor. follow the methods universally used The standard 'as-cast' exhaust The side of the combustion for Pinto engines. The valves are port exit measures on average chamber opposite the sparkplug is not enlarged to the maximum size possible 34.0mm/1.345in in height by touched during unshrouding work as it without altering the valve guide centre 24.5mm/0.970in in width. This must is far enough away from the valves not distance; the combustion chambers be opened out to 35.0mm/1.380in to cause any disturbance to gasflow. are unshrouded close to the cylinder high by 29.5mm/1.160in wide (the Adjacent to the inlet valve, the amount head gasket (but not so close as to size of the standard gasket aperture). of material removed from the combus- make the engine unreliable); the With an oversized exhaust gasket the tion chamber wall between 'A' and B exhaust ports are enlarged size of the exhaust port exit needs to (see photo opposite) is approximately considerably but the inlets are not; the be 36mm/1.420in by 30.5mm/1.20in lmm/0.040in, the amount of material valve throats are enlarged. These wide. removed between B' to 'C increases modifications produce very good At the point 32mm/1.25in into from lmm at 'B' to 3mm/0.120in at results at a reasonable cost. the exhaust port from the exhaust 'C,' between C and D the full 3mm The exhaust port outlets at the manifold flange, the modified port is removed and then, between 'D' and side of the cylinder head (where the should be 35.0mm/1.375in high and 'E,' the amount of material removed exhaust manifold fits) are opened out 1.150in/29.0mm wide (whichever type reduces to lmm. Adjacent to the