Jim Smart
March 30, 2007
Photos By: Benton Jackson

What's Up Top?
We don't give cylinder heads and valvetrains enough of our attention, but these are areas where engines fail most. Because valves and springs do one of the toughest jobs inside an engine, they mandate our every attention. Valve-stem-to-guide clearances must be spot on-not too tight and not too loose. MCE Engines recommends a window of 0.0015-0.0025 inch on stem-to-guide clearances. For intake, go with 0.0015 inch. For exhaust, push it more toward 0.0025 inch because the exhaust valve runs much hotter.

During assembly, Marvin uses Comp Cams assembly lube No. 153 over Dow Corning dry film lubricant No. 321 for ideal valve-stem lubrication for that first firing. Valve clearances can bite us in the butt if we're not diligent during assembly. Again-more of that mock-up stuff. Mock up your engine's cylinder heads and check clearances. How close are the valves to each other? How much clearance is there around the valve heads? What is piston-to-valve clearance? Have you checked all of them? Just because clearances are acceptable at one bore and chamber doesn't mean they will wash with the rest. No two chambers are the same, so don't expect them to be.

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