Critical Clearance: V-To-P
One 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.
 To check valve-to-piston clearance...  To check valve-to-piston clearance via the clay method, a lump of clay is laid into the valve notch area of the piston while the cylinder head is off. The head is then installed, along with the complete valvetrain, and the crank is rolled over through the overlap event. The head is then removed, and the valves' impression in the clay is examined and the thickness measured with a caliper to determine clearance. The clay may be sectioned with a razor blade for a full view. |  An alternative means of measuring...  An alternative means of measuring valve-to-piston is to rig the valves with light checking springs, and then installing the head and valvetrain with a dial indicator reading off the retainer. Start at 20 degrees BTDC, set the indicator to zero, and then work the exhaust rocker by hand until the valve hits the piston, and read the indicator travel. Repeatedly adjust the crank position 2 degrees forward, re-zero the indicator, and check again. The reading will diminish to the minimum V-to-P value, and then open up again. Next, perform the same procedure on the intake valve, moving from TDC to about 20 degrees after. | |
Bolting It Together
By 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.
 The cylinder head gaskets...  The cylinder head gaskets are an important part of the assembly. The gaskets must be capable of sealing at the power and pressure levels the engine will encounter, and be of the correct configuration for the engine. The gasket's thickness should provide the desired piston-to-head clearance, particularly when setting the engine up for a useful quench effect. These Cometics for a big-bore 572 Mopar were custom ordered for the bore size, and give 0.040-inch clearance to the narrow quench band of the custom dished pistons. |  Cylinder head fasteners should...  Cylinder head fasteners should not be neglected. Studs provide greater clamping load than bolts, but make head removal more difficult, particularly in the vehicle. Make sure that the fastener's engagement in the block is sufficient, and use sealant if the fastener goes into the water jacket. Torque the fastener to its manufacturer's specs, using the engine manufacturer's pattern. Cylinder heads are torqued in steps. Unless the fastener manufacturer specifies otherwise, I usually torque by first seating the fasteners, then work the pattern to 50 percent of the rated torque, then 75 percent torque, and finally, full torque. Be sure to lube the thread with the lube specified by the fastener manufacturer. ARP can supply new studs or bolts of very high strength material for most popular engines. |  The valvetrain is typically...  The valvetrain is typically more easily assembled with the intake manifold off. If the homework has been done ahead of time, the valvetrain will come together with no unforeseen surprises. Usually the valve covers wait until the intake has been installed, though they should be checked for adequate clearance by laying them in place without the gaskets, and rolling the engine over two full rotations and checking for contact with the assembled valvetrain. |
 Installing the intake manifold...  Installing the intake manifold shouldn't pose an undue headache, if the alignment was verified during the mock assembly. It is common practice to use a bead of silicone sealant on the end-rails where the factory used a separate end-rail gasket segment. Quality flange gaskets typically need no further sealant. Secure the intake manifold according to the intake manufacturer's specs. | | |