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Discussion Starter · #1 ·
I would like to make a piston stop for measuring TDC.
My plan is to hollow out an old spark plug and JB weld a suitable size and length bolt inside the threaded area. My question is just how far past the end of the plug would the bolt have to be to make contact with the pistons
1) for dish pistons
2) for flat tops

If I carefully turn the engine by hand and mark the crank pulley where the piston's travel is stopped then turn the engine the opposite way and mark this again the mid point should be TDC .... CORRECT???
 

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Actually the best way to measure TDC without messing up a piston is to use a dial indicator with an extension screwed onto the indicator and the body of the indicator threaded into a spark plug adapter. That way you can see TDC in thousandths of an inch.

Ron
 

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when we time mags on an airplane, we use a plug to find tdc. we first spin the prop forward to the plug, then backward to the plug (on the compression stroke). the midpoint is tdc.
 

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since the spark plugs are angled wouldn't that make the dial indicator bind because of the way the piston is pushing up on it, I know that works on engines with spark plug perpindicular to the piston such as snowmobile engines.
 

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jordan said:
since the spark plugs are angled wouldn't that make the dial indicator bind because of the way the piston is pushing up on it, I know that works on engines with spark plug perpindicular to the piston such as snowmobile engines.
Yes it will. I've never had consistant results using a dial indicator in an Opel's spark plug hole. I also tried the 'spark plug' method with a custom plug, and merely succeeded in bending my custom plug. It takes very little force rotating the engine to hit the stop and bend it.

I still prefer to remove the head and indicate off #1 piston (near the center to avoid piston rock), for true TDC. I admit this is not always convenient, but I do it with every new engine, and then mark the crank pulley. From there I use a degree wheel to mark the pulley in 5 degree increments up to 40 degrees.

HTH
 

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Discussion Starter · #7 ·
DIY TDC

Bob
I know this may be a pain ....but do you have any pics of one of your crank pulleys with the aformentioned degree marks to 40 degrees
 

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Bob, I have seen dial indicators with an angled extension on them and a little roller on the end for just that purpose, the angled spark plug hole. I think a motorcylce shop mech could point you in the direction of getting one if needed. HTH.

Ron
 

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Re: DIY TDC

azopelnut said:
Bob
I know this may be a pain ....but do you have any pics of one of your crank pulleys with the aformentioned degree marks to 40 degrees
Sorry Paul, don't currently have any pics.

Bob
 
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