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This is the most important step in the entire degreeing process. You must accurately determine TDC or all other measurements will be inaccurate. With the piston stop securely in place, carefully turn the engine clockwise until the piston contacts the stop. Record the degree wheel reading-in our case, it was 35 degrees before top dead center (BTDC). Now turn the engine counterclockwise until the piston again hits the stop. The degree wheel will be at true TDC when both piston stop numbers are the same. Let's say the numbers do not agree with 36 on one side and 40 on the other. Halfway would be 38 degrees, so move the wheel to that number and double-check your results.