Formula to shave pistons to reduce compression ??

Brooks James said

Is there a formula? 4.030 pistons 308 heads. It’s almost 11:1 want to go to 9:1. Use: pump gas and nitrous.

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Take them to a machine shop & have them milled to a dish... I haven't done the math on your engine but probably a 12cc or more dish will get you there. Problem maybe if they're milled & you're using nitrous you'll most like make a very large gas port about 3 inches around because they were to thin.
Sometimes we have to do things we don't want to, Lord knows I have had to buy something else more than once. Good luck.
You could get away with a .030 over factory 5.9 magnum piston (depending on a few things)