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Accurate top dead center question ?

Larry DeCamp

Well Known Member
Lining up the case split with 0 on the flywheel is OK, but why do folks debate advance numbers with such an arbitrary reference.

So my question is how to be precise about where is TDC. I fabricated a Stop that screws in the spark plug hole. It has a threaded stud to move in and out to contact the piston.

My question is if you just back off the stop so the piston will retract, not contact it, the DEADBAnD at TDC is significant. So, if you advance the stop until contacts the piston, you must rotate the crank 360 until it contacts again. But, this position includes the dead-band in clearances from reverse rotation, so the second datum should be marked by rotation of the crank back in normal direction, right ?

Then, split the difference between marks and that is effective TDC, right ??
 
This is essentially how TDC has been found for eons. Except with the addition of a weighted pendulum pointing at a 360* dial on the end of the spinner. Google "flower pot TDC indicator"
 
Lining up the case split with 0 on the flywheel is OK, but why do folks debate advance numbers with such an arbitrary reference.

So my question is how to be precise about where is TDC. I fabricated a Stop that screws in the spark plug hole. It has a threaded stud to move in and out to contact the piston.

My question is if you just back off the stop so the piston will retract, not contact it, the DEADBAnD at TDC is significant. So, if you advance the stop until contacts the piston, you must rotate the crank 360 until it contacts again. But, this position includes the dead-band in clearances from reverse rotation, so the second datum should be marked by rotation of the crank back in normal direction, right ?

Then, split the difference between marks and that is effective TDC, right ??

You are correct. Stop (mark)-unwind back the Stop (mark) then measure in-between.
 
You are reinventing the wheel.
https://www.aircraft-tool.com/shop/detail.aspx?id=E25
The flower pot timing indicator uses a stop to find TDC. Spin it to hit the stop, set the protractor, spin it the other way to hit the stop, note the reading, split the difference and set the protractor to that. That is TDC. Turn the prop to the setting you want, set the mags there and voila.

That said, the case split is as accurate on Lycs, as is the starter notch. I use my flower pot on my Continental and the starter on my Lyc.
 
I have to confess I didn't look this up until after I posted above and I'm absolutely shocked to see these selling for north of $60 bucks. I made one as a class project in A&P school back in the day for about $5 in parts.

Weighted pointer- made from aluminum scrap
Flower pot- I dunno, probably wal-mart
Index disc- photocopy glued to a aluminum disc cut with fly cutter
Couple of big rubber bands

Index pin- old spark plug with the center busted out and a thick piece of copper rod JB welded in the end.
 
Starter hole

There's also a pin hole on the starter. Stick a probe in there and the flyweel is dead on TDC.
Or is that mark and hole for something else?
 
Got a down leak compression tester? You can use the spark plug adapter as a guide. Find the compression stoke with your finger over the opening and get it semi close to TDC. Insert a stiff wire/filament/etc through the adapter. Feel the movement of the wire relative to the adapter when you move the prop. You can find TDC to within a fraction of a degree. Closer than mag gear lash. Try it and see.
 
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I have to confess I didn't look this up until after I posted above and I'm absolutely shocked to see these selling for north of $60 bucks. I made one as a class project in A&P school back in the day for about $5 in parts.
...
Get ready to be double-shocked!

https://www.aircraftspruce.com/catalog/topages/timerite.php

It look like a pretty cool tool, I have to admit, probably a good candidate for a local chapter to put into the tool crib.
 
Not too long ago timed a R1340 and used a bent piece of music wire zip tied to a dial test indicator probe with the magnetic base attached to the steel part of the cylinder. Crude but worked far better than using a borrowed Time Rite. Once TDC was found I rubber banded and taped a sensitive angle meter to the prop. Could use a phone and an angle app.
 
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Not too long ago timed a R1340 and used a bent piece of music wire zip tied to a dial test indicator probe with the magnetic base attached to the steel part of the cylinder. Crude but worked far better than using a borrowed Time Rite. Once TDC was found I rubber banded and taped a sensitive angle meter to the prop. Could use a phone and an angle app.

I’ve used the phone taped to the prop on Continental before , they are not as easy as the Lycoming
 
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