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Need to find true TDC?

Try this.

Take an old spark plug (preferably one that fits the engine) and break the porcelain and remove, leaving the steel shell.
Cut threads to the I.D. of the shell with a 3/8 x 16 NC tap.
With a hacksaw cut a slot on one end of a 3/8 x 2 NC stud to fit the tip of a flat bladed screwdriver.
Now screw the stud into the newly threaded spark plug shell.
You now have a Piston Stop.
To use you’re new tool;
1) Disconnect battery.
2) Turn the crank to somewhere before TDC (approx. 30 degrees)
3) Screw the tool into No. 1 plug hole.
4) Now slowly turn the crank clockwise until the piston comes up and contacts the tool. (You may have to adjust the depth of the tool with a screwdriver to insure full contact).
4) Now that the piston is stopped against the tool, mark the harmonic balancer at the TDC timing mark or create a reference timing mark on the block
5) Now, rotate the crank counter clockwise until the piston comes up again and contacts the tool again. Mark the harmonic balancer again at the TDC or new reference timing mark.
You now have two new marks on the balancer.
Divide the two marks in half and that result is true TDC.

PS. Don’t forget to remove the tool.