How To Check Valve-To- Piston Clearance

We cut the intake valve clay impression in half with a razor blade to easily check the amount of intake valve-to-piston clearance. We had in excess of 0.200 inch on the intake side, so we have no problem here.

The first step is to position some modeling clay (Play-Doh also works-your kid won't mind) over the intake- and exhaust-valve pockets. Remember to use the same thickness head gasket as the one you will ultimately use. Spreading a tiny film of oil on the valves will prevent the clay from sticking.

Yank the head. This will reveal the indentations left by the valves. You can use the long end of a Vernier caliper to measure the thickness if you like, but at some point, take a minute to carefully study the indentations and the relationship between the valves and piston.

Next, we tried a much larger aftermarket aluminum big-block cylinder head with 2.300/1.88-inch intake and exhaust valves. The first thing we discovered was that the large intake valve hit the piston just before the piston reached top dead center (TDC). We hand-filed the area, but this moved the interference to the crown of the valve relief in the piston. This would require milling larger valve reliefs.

To mark the intake- and exhaust-valve centerlines in the piston, JGM uses a tool made from an old valve that acts as a transfer punch. We set the piston at 10 degrees before top dead center (BTDC) and punched both valve centerlines in the piston. Any good machine shop can then use these centerlines (along with the stock valve angle) to fly-cut all eight pistons for larger valve reliefs.

Here we used a Vernier caliper to simulate the radius of a 2.300-inch intake valve. While in this photo it appears the larger valve will clear, it in fact hits all along the top of the piston relief crown.