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Discussion Starter #1
50 years ago when I had a 1954 AJS, we used a dial indicator to go through the spark plug hole to indicate when the piston was at TDC.



We that are interested on performance want to know the real TDC, not what the marks indicate. They could be wrong.



How many of you would buy one of these?



I'm ready to buy one.



Don
 

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Discussion Starter #5
I too have built a few of them.



It is easy in theory, but not so easy in practice for these engines.



The problem is the angle of the spark plug. The angle is so shallow that the indicator wants to bend instead of deflect. You really need a bent tip (in a bent tube) on the end so that when the pistons pushes the indicator retracts instead of bending.



It only took me a half hour to adapt one for my dial indicator, but you would need a friction free one to work good.



So which commercial ones work good and which don't.



So which ones are in your tool box and which ones are in the trash bin?



How much are the marks off on your engine?



Don
 

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Discussion Starter #7
firepower354 said:
I prefer the solid stop, turn both ways, split the difference method. Too much dwell at TDC to be accurate with a dial indicator.


I do that with a dial indicator too.



I guess I'll build a LOOOOONG reach spark plug.



Don
 

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I used to use one my dad made, to set the points on my GTMX80 and TY250, back when they spec'd distance BTDC for timing. The poor angle of most mini head plug holes makes it a bit iffy, like you said.
 

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firepower354 said:
I prefer the solid stop, turn both ways, split the difference method. Too much dwell at TDC to be accurate with a dial indicator.
That is the ONLY wat to get true TDC.
 

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T Bolt USA said:
Or Pull the head...


Which, unless you use a bolt on stop that the piston hits before TDC, and roll in both directions, mark and split, exactly as the plug hole one would be used, is just as inaccurate as a screwdriver in the plug hole. Dwell at TDC isn't accurate with a dial indicator, and sure as hell isn't with an eye-crometer:rolleyes:
 

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Since these are just pitbikes and not formula one race engines, 99% of those here will be fine using that little"T" mark on your flywheel.

And a dial indicator if read and used propely will work very well at finding TDC.



The same way as the "stop" method, just pay attention and split the difference.
 

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All due respect to Geezer, that is not actually the only true way to do it

To use a dial indicator, and note the pointer reading at about 1mm before tdc, and 1mm beyond tdc, measured in piston travel. Then the pointer position exactly halfway between these readings is exact TDC.
 

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Matified said:
All due respect to Geezer, that is not actually the only true way to do it

To use a dial indicator, and note the pointer reading at about 1mm before tdc, and 1mm beyond tdc, measured in piston travel. Then the pointer position exactly halfway between these readings is exact TDC.
I thought that was what he said. I build drag race engines and use the piston stop-both directions,and split on degree wheel. also use [email protected] when degree cams.On race car engines that is. Im in the 1%.Sure you can get really close shoving a wire in the hole and see how far it comes out too.Its all good.I dont know these engines-Yet.Or how close factory is what i mean. Is there something i missed on the piston stop split method?
 

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geezer said:
I thought that was what he said. I build drag race engines and use the piston stop-both directions,and split on degree wheel. also use [email protected] when degree cams. Im in the 1%.Sure you can get really close shoving a wire in the hole and see how far it comes out too.Its all good.I dont know these engines-Yet.Or how close factory is what i mean.
I guess FPM was trying to say a dial indicator was not accurate for finding TDC, and only the stop method was the end all , I agree that is a solid method and most practical.

Since the OP was referring to a dial indicator, I was confirming that , Yes one can find TDC with one properly set up and used.
 

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degree wheel and tdc stop is the quickest method, as many have already agreed and posted. The fancy stuff tends to look good but just adds unnecessary details.





Isn't "significant error" letting your beer get warm while trying to reset your dial indicator? lol
 
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