TDC

T

Tom

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Guys, i know there has been questions within this topic has been asked many times in the past but i have a question regarding TDC and piston stops.

I've bought a piston stop suitable for finding TDC but find that it's hitting the valves. Can i just loosen the valves off so there's no interference? Unlike a piston stop made out of a spark plug, my piston stop can screw in quite far. How far should i screw it in?

Any help would be greatly appreciated.
 
Turn the crank one full revolution, now use your gauge, on this stroke the valves won't be opening before or after TDC
 
This is my piston position sensing tool.

TDC-Finder09.jpg

At that length, it just barely touches the piston at TDC.
1/16" longer than that will suffice...
 
My particular specific measurement may not help you, since various models had different compression ratios, piston heights, gasket thicknesses. That's why good piston stops have adjustable plungers.

But, just to grab a number out of the air, let's say 1-1/2" total, threads plus plunger...
 
Last edited:
Back the valves off and just experiment. You can watch and see the piston get close to TDC When you are close thread the stop in with the plunger backed off and slowly turn it in untill the plunger hits the piston top. Thy it there and see what happens. I probably don't have to tell you that everything you do should be extremly slow a deliberate to avoid causing any damage.
 
Three ways to get 'er done.

1. Most accurate: Dial indicator with long shaft. With piston rising on compression (valves closed), take degree readings at equal drop BTDC and ATDC. Raise the shaft while the piston is on the rise to prevent binding due to side load. True TDC is the midpoint of the degree readings.

2. Pretty accurate: Piston stop with movable plunger and short shaft dial indicator or horizontal dial test indicator (preferred). Same drill.

3. Close enough fer gummint work: Piston stop with movable plunger and scribed lines to gauge depth. Same drill.

However you do it, make sure that your measurements repeat.
 

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