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Using a "bridge" with a dial indicator is the best way to figure true top dead center. Starting with the piston a few thousandths of an inch before TDC, check the degree reading on the wheel, at say "10" for example. Rotate the engine so the piston is the same depth after TDC and record the degree reading there, say at "5." Subtract the smaller number from the larger (giving us 5) and divide that in half (2.5 degrees). That is how many degrees you need to move the pointer over from "zero" toward the "larger number" side and land it on true TDC.