Proper piston-to-valve clearance is the difference between a winning-engine and an expensive pile of scrap metal Here's how to check clearance using clay.
The relationship between the valves and the pistons is a precarious one. Too little space and they collide; an occurrence which results in carnage that, at the minimum, requires replacing the valve(s), guide(s), and piston(s). At a maximum, it necessitates a complete engine rebuild. Nobody wants to pick chunks of steel and aluminum out of an oil pan.
Piston-to-valve clearance criticality is exacerbated by 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 motor and a sad wallet.
The guinea pig for this story is a big-block Chevy being built by CNC Motorsports. The cylinder heads and pistons being checked for clearance are from Brodix and JE Pistons respectively.
The first step is to check the uncompressed thickness of the head gasket and write it down. 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.
A handful of Play Doh, or similar modeling clay, is flattened out across the valve reliefs on top of the JE pistons (about 1/4in thick). A light coat of engine oil or assembly lube on the clay, combustion chamber, valves, and cylinder bores prevents the clay from sticking and giving a false reading.
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 pretty snug. Assuming both decks are of equal height, the checking process only needs to be done on cylinder one.
Even if the engine will use hydraulic lifters, you must use solid lifters to check the clearance. Hydraulic lifters may bleed down during the checking process (even with a soft checking spring) and affect the measurements. If substituting solid rollers, make absolutely sure that the height of the lifter’s pushrod cup is the same as what you’ll be using in the completed engine. Set the valve lash to zero by rotating the pushrod between your fingers while tightening the rocker arm; when you feel resistance to turning the pushrod, that’s zero lash.
Using a ratchet and socket, 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.
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.
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
