Valve lash is the deliberate clearance between the rocker arm and the valve tip in a solid-lifter engine, set with a feeler gauge so the valvetrain still has room when the engine heats up and grows.
The number comes off the cam card. EngineLabs notes that “all camshaft companies list the operating clearance — or lash — in the specs,” and those specs are written “for a hot engine, or one that has reached its normal operating temperature.” That’s where the argument starts, because most people set lash cold, in the shop. Correcting cold for hot isn’t one number and it changes direction with what the engine is made of. Crane’s chart, via EngineLabs: iron block and iron heads, add 0.002-inch; iron block with aluminum heads, subtract 0.006; all aluminum, subtract 0.012 — “aluminum expands at nearly twice the rate of cast iron or steel.” What nobody agrees on is how hot “hot” is. Ask your builder what the card assumes.
Lash set too tight can hold a valve off its seat on a cold start and burn it, which is exactly why the cold number and the hot number are not the same number.
Read the full post:
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