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To illustrate the piston-to-valve clearance, we mocked up the engine the old-fashioned way by putting clay in the valve reliefs. Duration is more critical than lift here, and in general, the intake valves are closest to the piston at 9 to 11 degrees after TDC on the intake stroke, as the piston is falling and the intake is opening; the exhaust valves are closest at 9 to 11 degrees before TDC on the exhaust stroke, as the piston is rising and the exhaust is closing. In this case, the problem wasn't the depth of the notches, it was the radial clearance between the relief and the edge of the oversize valve (arrow). View Related Article
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