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Engine Building Fundamentals: Measuring Piston to Valve Clearance

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Measuring Piston to Valve Clearance


00:00 - While there are a variety of ways of confirming or measuring valve to piston clearance, my own preferred technique is to place a strip of Play-Doh or modelling clay across the valve pockets of the piston, with the engine shortblock assembled.
00:15 Before doing this, it's advisable to clean down the piston to remove any traces of oil, as we want the Play-Doh or modelling clay to stick to the piston, and not the valve.
00:26 I'll also press the Play-Doh roughly into the valve pockets at this point.
00:31 It's easy to confirm the clearance of both the intake and exhaust valves in one test, by applying the Play-Doh or modelling clay to both sides of the piston simultaneously.
00:43 Before installing the head, I'll apply a thin coat of engine oil to the top of the Play-Doh, to prevent it sticking to the valves.
00:51 Now we can instal the head and rotate the engine through two full revolutions.
00:57 While doing this, it's important to move slowly, and stop if you feel any resistance, as it may indicate that the valves are making contact with the piston.
01:07 Before performing this test, it's important to ensure that the cam, or cams, are dialled in, or degreed correctly, since cam timing affects the timing of the valve opening and closing, relative to the piston position.
01:22 It's also important to use all of the valve train components, and the same head gasket that you intend to use for the final assembly.
01:31 It's common to perform these mock-up tests using light test springs instead of the actual valve springs that will be used in the final assembly, and doing so can affect the accuracy of your results.
01:45 Once the engine has been rotated through two complete revolutions, the head can be removed, and the impressions in the Play-Doh or modelling clay can be inspected.
01:55 Initially, I inspect the Play-Doh to ensure that it's still intact, and that there's no immediately obvious sign of contact between the valves and the piston.
02:05 If this looks OK, I'll use a sharp knife or razor blade to cut through the Play-Doh, in line with the centre of one of the valves.
02:15 This will give us a cross section of the clearance remaining, through the centre of the valve pocket.
02:21 The thickness of the Play-Doh indicates the clearance, and using a set of vernier callipers, this can be measured.
02:28 The thickness of the Play-Doh at the edge of the valve pocket, can be used to indicate the radial valve clearance.

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