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I installed one of these 6:1 heads a couple years ago. I cleaned the block surface of all old gasket material then placed the head on the bare block. The head was warped about 0.013"! They offered to send me a new head (probably also warped) after I paid return shipping and waited another three weeks. I suggested machining it locally and they pay the bill. They agreed and everything was installed. The machine shop verified my crude measurements. That "premium" recommended head gasket with the silicone failed shortly. Replacing with a copper gasket solved the problem. I have to also play with spark advance on heavy pulls to keep the spark knock from happening. I read somewhere about the need to retard timing a bit when adding compression and I am presently timed like a stock engine. I like the power and the mileage and I'm sure the old original head would be a pain to live with now.