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"We are checking the piston-to-valve clearance with all of the valvetrain installed. We put a dial indicator on the retainer and roll the engine over to 10 degrees before TDC for the exhaust and 10 degrees after TDC for the intake, zero the dial indicator, and open the valve with the wrench. You can then see how many thousandths until the limit. The most accurate way to check the clearance is by using all the springs and the pushrods. By mocking up the engine this way we can maximize the piston dome design when we final cut the valve pockets." said Pettis. A checker spring won't deflect the valvetrain realistically resulting in inaccurate measurements; this is done during the engine mock-up phase.