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Engine – Valve to Piston Clearances

Anytime a modification is made to a cylinder head such as milling or decking, increase in valve size, larger camshaft or valve train components, it is extremely important to check and establish piston-to-valve clearance. The primary reason is to make sure that the pistons and valves do not hit each other. Contact between a piston and valve can cause extreme engine damage or complete engine failure. Piston-to-valve clearance can be measured in several ways. A common method, also know as the "Clay" method, can be performed and will be discussed in detail for this newsletter. LC Engineering recommends a minimum of 0.080" (eighty thousandths) for piston-to-valve clearance on most applications.

With the cylinder head removed, spread modeling clay evenly across the top of the #1 piston with the piston at TDC. Place the cylinder head; with the #1 intake and exhaust rocker arm lash set at 0 (zero), on top of the block using a crushed head gasket. Secure the head in place by tightening the head bolts/studs so they are snug. It is not necessary to torque the cylinder head down to the block since you will be removing it after this next step.

Connect the timing chain to the upper sprocket with the cam set at the correct timing. Now rotate the engine slowly clockwise, by hand, a single revolution back to TDC. Loosen the head bolts/nuts and remove the cylinder head from the block. You should now see some impressions in the clay from the valves. Using an Exacto Knife, slice the clay from top to bottom where the intake and exhaust valves left an indentation. With a set of calipers or depth gauge, measure the thickness of the clay to determine the correct piston-to-valve clearance number. Again, the number should be no less than 0.080".

NOTE: It should be relatively easy to rotate the engine clockwise with a ratchet attached to the crankshaft. If you feel any interference, or the engine will not rotate any further, STOP!! Any amount of force placed on the valves or piston will cause damage. Most likely the pistons will need "notching" in order to continue with assembly.

Clay spread evenly across top of piston before head is placed into position.

After rotation of the engine, and removal of the head, you should see an indentation in the clay.

Use a knife to slice through the clay to establish an area to measure with.