Critical Clearance: V-To-POne of the most important checks to make when building an engine is the valve-to-piston clearance. As cam duration and compression ratios go up, this figure becomes more critical. Some make the mistake of presuming that the valve-to-piston clearance is a function of the cam's lift specification, but in reality it is more related to the duration and lobe separation angle. The piston is well out of reach of the valve by the time the valve reaches maximum lift, usually at around 102-112 degrees after TDC. The critical factor is what's happening during overlap, when the piston is very near TDC, the exhaust valve is closing, and the intake valve is opening. The exhaust valve will normally get closest to the piston somewhere in the vicinity of 15 to 5 degrees before TDC, while the intake valve will be closest in about the same range after TDC.The most common methods of checking valve-to-piston clearance are directly reading the travel to contact with a dial indicator or making a check using clay on the piston tops. Both will provide a good indication. The dial indicator system will give a more precise number, but does not reveal potential radial clearance problems of the valve's edge to the position of the valve pocket machining. The clay system will show exactly where the valve is in relation to the piston and valve notches, but it is more difficult to get an exact clearance number.
Bolting It TogetherBy now the short-block should have the cam installed and degreed, and the cylinder heads should be standing by fully assembled, with the compatibility and clearance of all of the valvetrain components verified. The piston-to-head clearance should have been verified during the short-block mock-up stage, and the appropriate head gaskets sourced for the desired quench clearance. All that is left is to bolt together the final long block by installing the cylinder heads, valvetrain, and intake manifold. These procedures are relatively straightforward if all of the checks have been made beforehand.