Like a chance meeting between a jealous wife and a flirty ex-girlfriend, having your pistons and valves come in contact can be very bad—and possibly destructive. That’s why it’s important to check piston-to-valve clearance, especially on racing camshafts and aggressive, high-lift street cams.
The camshaft and valvetrain experts at COMP Cams put together this easy, seven-step process for checking piston-to-valve clearance using the tried-and-true clay method. As a general rule of thumb, we’d recommend a minimum of .080-inch clearance on the intake valve and .100-inch on the exhaust side. Keep in mind, too, you’ll need to check for radial clearance around the valves within the valve pockets.
Before torquing down that last head bolt, use COMP Cams’ outline below to check for proper piston-to-valve clearance.
With the camshaft installed and degreed, remove the cylinder head from the block. Clean the combustion chamber and the top of the piston and valve reliefs—the cleaner the piston, the better the clay will stick to it.
Apply a strip of model clay 3/8-inch to 1/2-inch wide and approximately 1/4-inch thick to the pistons. The clay strips should be placed perpendicular (across) to the intake and exhaust valve reliefs. By applying a small amount of oil to the clay or bottom of the valves, you can prevent the clay from sticking to the valves as they are pressed together.
Reinstall the cylinder head with the gasket that is going to be used. All head gasket manufacturers can tell you what the compressed thickness of their gasket will be, so it will not be necessary to re-torque the head completely. Simply measure the gasket before you install it permanently and add the difference of the gasket thickness to your piston to valve clearance. This will be within .001-inch or .002-inch of the exact clearance. Install a sufficient number of head bolts to secure the head in place while you are rotating the engine. Install the pushrods, lifters, and rocker arms on the cylinder you have prepared for the clearance check.
Adjust the rocker arms to their suggested clearance. If the camshaft you are checking uses hydraulic lifters, you must temporarily use solid lifters in their place as hydraulic lifters bleed down and would create a false measurement. Once the hydraulic lifters are replaced with solid lifters, adjust the lash to “zero.” Be sure not to pre-load the valve spring. (IMPORTANT NOTE: Be sure to reinstall the hydraulic lifters before starting the engine).
Turn the engine over by hand in the normal direction of rotation. Be sure to rotate the engine over two times. This will be one complete revolution of the cam and it will assure you of an accurate reading on both the intake and exhaust. As a general rule, the exhaust valve is usually closest to the piston at 10 degrees BTDC, and the intake valve is usually closest to the piston at 10 degrees after top dead center (ATDC). By advancing and retarding the timing, you will affect the looseness or tightness of the tolerance. Remove the cylinder head from the block. Be sure to do this gently, so the clay is not disturbed. It may be stuck to the valves or combustion chamber, so be careful.
With a razor or sharp knife, slice the clay cleanly lengthwise (see left) through the depression and peel half of it off the piston. The clay’s thickness in the thinnest area will represent the minimum piston to valve clearance.
To accurately check the thickness, use a set of dial calipers.