In an engine, the relationship between the valves and the pistons is a precarious one. If there is too little space, they can collide. If this happens, the resulting carnage require replacing one of the valves, guides, or pistons. At a maximum, it necessitates a complete engine rebuild. Nobody wants to pick chunks of steel and aluminum out of an oil pan.

Proper piston-to-valve clearance is especially critical in applications with high-compression pistons, large-duration cams, and high rpm. For these reasons, it's imperative to check piston-to-valve clearance whenever an engine is built, or either the cylinder heads or valvetrain are changed. Checking this all-important clearance is not difficult and can pay huge dividends in durability. The shortest route to disaster is assuming everything is OK.

In order to check clearance, you will need Play Doh, or similar modeling clay, an inexpensive dial caliper and a pair of solid lifters. As for the engine itself, the short block needs to be finish-assembled, you'll need the cylinder heads, head gaskets, and the camshaft/valvetrain that will be used. The process, as detailed in this article, is simple enough and could mean the difference between a happy engine and a sad wallet.

Valve Clearance Tips

Recommended valve-to-piston clearance is generally 0.080-inch minimum on the intake side and 0.100-inch minimum for the exhaust valve. The exhaust needs a little more clearance because it expands more with heat.

Add 0.030 inches of minimum clearance to each valve if the engine is equipped with aluminum rods since they tend to stretch more than steel.

Most piston to valve issues happen at 10 degrees before top dead center (BTDC)

A few degrees of advance or retard at the camshaft can radically alter the piston to valve clearance. Make sure piston-to-valve clearance is checked with the cam installed in the exact position it will be run.

Clean the top of the piston and valve reliefs. The cleaner the piston, the better the clay will stick to it during checking.

Carbon buildup on the piston and valve faces will affect clearance.

See all 10 photos

See all 10 photos

See all 10 photos The first step is to record the uncompressed thickness of the head gasket. Then, find the manufacturer's listed compressed thickness for that gasket and subtract it from the uncompressed thickness measured on your gasket. Write this number down as it will be important later.

See all 10 photos A handful of Play Doh, or similar modeling clay, is flattened out across the valve reliefs on top of the pistons (about 1/4-inch thick). A light coat of engine oil or assembly lube on the clay, combustion chamber, and valves prevents the clay from sticking and giving a false reading.

See all 10 photos Bolt the gasket and head into place and install the valvetrain for the cylinder being checked. You don't have to fully torque the head bolts, but they should be snug. Assuming both decks are of equal height, the checking process only needs to be done on one cylinder.

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See all 10 photos Rotate the crankshaft for two complete revolutions. This will be one complete revolution of the camshaft (the cam spins half the speed of the crank) and assure you of an accurate impression in the clay from both the intake and exhaust valve.

See all 10 photos Loosen the rocker arms and remove the pushrods, remove the head bolts, and carefully remove the cylinder head, making sure it's not sticking to the clay. This is where the smidge of the oil comes in handy.

See all 10 photos

See all 10 photos With a razor blade or knife, cut the clay in the middle of each valve relief and peel one half off the piston top. Measure the thickness of the remaining halves with a dial caliper. Check one valve at a time and write down the measurements you come up with. This is not the final piston-to-valve clearance. To find the true piston-to-valve clearance, use this formula:

Clay Thickness (Gasket Uncompressed Thickness - Gasket compressed thickness) = PTV Clearance

Example 0.130-inch (0.040-inch - 0.035-inch) = 0.125-inch clearance.