Top Dead Center finder

[Flying Scotsman]

On a whim, I bought something called a Piston Stop for determining TDC, from ACS

http://www.aircraftspruce.com/catalo...clickkey=44593

Not that I *need* it or anything, because the starter ring gear has timing marks and it's easy enough to find the compression stroke on cylinder 1, anyway, but...figured what the heck? I like tools...

Maybe I'm missing something here, but this sucker doesn't seem to be correct at all...at TDC on #1, the thing won't even go in deep enough to the spark plug hole to thread. Too long.

Am I doing something wrong, or is this just a bogus deal that nobody ever uses anyway? Or are there different sizes of these dudes? What the...?

Just curious if it's at all useful before I send it back...

[BobTurner]

As the piston comes up to TDC it stops moving while the crank is rotating. Point is, the accuracy is poor if the finder was just long enough to touch at TDC. Here's how you're supposed to use this:
Attach a plumb bob and a protractor to the spinner. Put the prop at bottom dead center. Install the finder. Slowly turn the prop in the usual direction until the piston contacts the finder. Read the protractor angle where the plumb bob goes by. Now turn the prop the wrong way, until the piston again contacts the stop. Read the protractor, where the plumb bob string goes by. Remove the finder stop. TDC is half way between the two angles you measured. Turn the prop to that angle, and mark the flywheel or other spot. If you have a digital level you can tape that to the prop and use it instead of the plumb bob.

[Flying Scotsman]

Ah HA! Got it now...doh!

I was thinking it was supposed to touch AT TDC, but I see now how it's supposed to work.

Thanks! (Like I said, though, I think it's probably a wash for me, since my Lycoming gear ring has the marks on it, anyway)...

[s24789]

A friend off ours bought one and used a digital protractor attached to the spinner. We were having CHT issues and found one mag was off by almost two degrees. It works great and solved our CHT problems.
Phelps