Finding Top Dead Center on an engine is a four-step process made easy with a piston stop and a degree wheel.
Step 1: Install piston stop across bore, rotate the engine until the piston touches it and zero out the degree wheel on the pointer.
Step 2: Rotate the crankshaft backwards until the piston comes back up and touches the stop. Note the indicated number on the degree wheel. (We got 80.)
Step 3: Divide that number by two and rotate the degree wheel (not the engine) to the result. (Our case would be 40)
Step 4: Remove the piston stop and roll the engine back around until the pointer hits zero. This is your Top Dead Center.