Stop in for a cup of coffee

krazykuda said

If properly machined and pressed in, the seats won't fall out... John machines the proper size hole and uses dry ice to cool the new seat inserts before pressing them in so they go in better and have the proper interference so they don't come out....

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That’s not the issue, it’s the MDS. The head temperature differential on those engines caused the valve seats to drop after cruising in MDS mode. The later heads had the valve seats pinned in so it wouldn’t happen. It was an engineering failure of the initial design.