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You can make your own tool to find true top dead center...

by using an old spark plug that fits that head.

  1. Remove the ceramic section of the plug by grinding or filing away the retaining collar that holds the ceramic body at the top of the metal body.
  2. Drill and tap the metal body to accept a 2 or 2-1/2 inch threaded stud.(most spark plugs are close to using a 3/8 NC tap)
  3. With a hacksaw, cut a slot at one end of the stud to fit a slotted screwdriver and thread the stud into the modified spark plug base.

You now have a tool that is an adjustable piston stop for determining Top Dead Center.


                           tdc 2              tdc 3             tdc 4

How to use;

Turning the crank clockwise, bringing No.1 cylinder up to within a half inch of its full travel.

Install the tool into the spark plug hole and adjust it until it just contacts the piston.

Now, turn the crank counter clockwise to lower the piston slightly and then:

1) Turn clockwise until the piston comes up and contacts the tool.

2) Mark both the harmonic balancer and the block as a reference point.

3) Turning the crank counter clockwise until the piston comes up again and contacts the tool again.

4) Make another mark on the harmonic balancer using the mark on the block as a reference.

5) Divide the two marks on the harmonic balancer in half and mark this as True TDC.

6) Remove the tool and turn the crank until your new TDC mark and your block reference mark line up.


That is True Top Dead Center.

I've used this to check harmonic balancers (with timing marks) where the inertia ring was suspected of moving and timing components without having to do a full disassembly.




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