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The rule of thumb is 0.004 in. for each inch of bore diameter. So figure a 0.016-in. minimum gap for a 4.00-inch bore. Testing has shown no compression loss or increase in blowby if you double that figure, but I wouldn't do that. But to ensure the ring ends don't butt, use a 0.020-inch ring-end gap.
I hope that helps.
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Tom
"If you can make black marks on a straight from the time you turn out of a corner until the braking point of the next turn, then you have enough HORSEPOWER." Mark Donohue
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