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Old 07-07-2017, 12:28 PM   #21
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Old 07-07-2017, 12:38 PM   #22
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Originally Posted by djmarchand View Post
Milling the head will have negligible effect on compression. But if you take off more than 0.009" or maybe 0.015 if you believe Bomac, then I do believe you have a risk of valves interfering with the piston. The consequences will not be good.

You could put a little putty on the top of the piston on one cylinder and lightly oil the valve head to keep it from sticking, assemble and tighten the head and then rotate the crank, remove the head and check the clearance. The putty should have depressions made by the valve heads and you can cut a groove with a razor blade to see how much putty is left.

What is the accetable spacing then?

RickB suggested the putty trick too....

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Old 07-07-2017, 02:45 PM   #23
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The gasser racing guys say this:

"So, how much clearance is enough? A safe rule of thumb is .080-inch for intake valves and .100-inch for exhaust, which requires a little more clearance because of the increased heat factor. Experts also suggest adding .030 to each valve if the engine is equipped with aluminum rods as they tend to stretch more than steel."

That advice may be wildly conservative as hot rodded gassers can run at 3 times the rpms that your old Lehman does. It is high rpm that stretches the piston rod and decreases clearance as well as high exhaust temps that expands valve stems. They do have a much lighter piston though, so who knows -.

Go to and read about it yourself.

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Old 07-07-2017, 03:17 PM   #24
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If you grind the seat to get valve face depression into spec, all will be fine. Might not even need to do that. Just check the measurement after the shave. In spec, all good.

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