Piston to Valve and Cylinder Head Clearances

Piston to Valve and Cylinder Head Clearances

Piston to valve clearance should be a minimum of .100". This clearance will be changed if heads or block have been machined and must be checked at assembly. While some people claim to “get away” with less clearance, there are many others that have bent valves and broken engines trying to do so. When using steel rods, the minimum clearance between the piston and the cylinder head should be around .040", aluminum rods require an additional .010 – .020" due to their tendency to stretch at higher engine speeds. Many engines use a flat “quench” area on the piston, which creates beneficial turbulence within the combustion chamber by coming into close contact with the bottom of the cylinder head. In applications having this “quench” design the clearance between the piston and the head should not exceed .060" in this area, or destructive detonation may occur. This is the reason that stacking head gaskets to lower the compression ratio usually delivers poor results.
Piston skirt strength and required bore clearance depend on material, skirt cross section shape, oil ring groove drainback design and where the clearance is measured on the piston. Stock replacement and moderate performance type pistons, whether forged or cast, use slots to return oil that is scraped from the cylinder walls by the oil ring. This design allows the skirt to be more flexible and permits the tighter cold bore clearances. Forged pistons with the slot design can be set up at nearly the same clearances as cast pistons. A high performance race type piston will use “windows” or drilled holes for oil return. The drilled hole design adds significant structural strength to the skirt of the piston but requires greater bore clearance since the skirt is less flexible and the amount of heat transferred from the piston head is increased

Contrary to statements from other manufacturers, the greater clearances do not cause ring sealing problems, as the working clearances are nearly the same once the piston reaches normal operating temperatures. These piston skirts are specially shaped to reach optimal clearances once warmed up, through careful attention to skirt cross section design and piston growth patterns. When cold, these pistons exhibit an oblong or “cam” shape around the skirt area. While a drilled hole forged piston may exhibit some noise when cold, it would be rare to hear a racer complain about it. Also, the place where measurements are taken is important. Since piston diameter varies from the pin bore to the bottom of the skirt, it is possible to have two pistons with different specifications but identical operating clearances. Skirt Taper See Catalog For Dimensions Measure Skirt Here Do Not Measure Here on Slipper Skirt Pistons