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9 Lapping valves is an old technique that helps match a valve to its corresponding seat for a tighter combustion chamber seal. Even though modern machining and production mean there's less tolerance between the two, lapping in the valves helps ensure the combustion chambers stay as sealed as possible. Keeping the valve seated against the seat, use a spinning motion to create the grinding action necessary. You'll feel resistance lessen as the two surfaces begin to mate with each other. Once the valve is fully lapped, wipe down the valve and seat to remove all lapping compound.