The problem is that the expansion rate of aluminum is greater than the expansion rate of the steel valve seat insert. The aluminum head expands and the valve seat just falls right out if the machining tolerance isn’t dead on. The high end solution is copper beryllium valve seats that actually expand more than the aluminum so they really lock themselves into place.
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1962 Biscayne O-21669 MKIV/M-22
1962 Bel Air Sport Coupe 409/1,000
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