http://www.enginebuildermag.com/2012/01/valve-seat-selection-finishing-materials/

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Chrysler has reportedly had problems with the exhaust valve seats loosening up and dropping out in some of its late model 4.7L engines. Seat failures have typically occurred between 60,000 and 80,000 miles, often after an overheating episode in which the coolant got above 250 degrees F.

Some engine builders say Chrysler didn’t use enough interference fit for the seats in these engines and recommend using an additional .002" of interference when the seats are replaced. Cutting the seat recess slightly deeper can also improve seat retention.

Some vehicle manufacturers use only a couple thousandths of interference when they install valve seats in a new cylinder head. They can get by with minimal interference because the heads are brand new castings that have been freshly and accurately machined. There’s been no wear, corrosion or motion to loosen up or distort the valve seat recess as is often the case with high mileage cylinder heads.
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