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Larry DeCamp

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I am fabricating a plunger for dial indicating accurate TDC to set Pmag timing. It would be simple to sleeve a spark plug and insert a round tip or roller tip push rod to sense the piston. Does anyone know how to get the insulator out of the steel spark plug body ?
 
Wow, this seems like the hard way to accomplish this task. I use a brand new traditional yellow wooden pencil with a rubber eraser on the end. I know somebody will say the rubber eraser may come off - that's why I use a new one that hasn't seen any in-service abuse. I keep this fancy and expensive tool in the same box as my compression tester and mag timer. The same pencil has been doing compression checks on several aircraft for at least the last decade and it doesn't even have a scratch on it.

And yes, you really can feel when the piston is at TDC if you are paying attention to the movement of the pencil.
 
TDC

I believe he is referencing finding the true TDC. You can push the center out with a light application of a press. Just push center electrode out from the piston side.
 
"True TDC"? Educate me, please! I always thought TDC was when the piston is at the top of its stroke, easily sensed by feel to within a fairly small margin of error of crank rotation. What am I missing?

If one really wants to use a dial indicator, what would be wrong with using the compression tester spark plug adapter - it's already hollow, and, with the air fitting attached, provides a good guide for a pushrod.
 
Canadian_JOY said:
"True TDC"? Educate me, please! I always thought TDC was when the piston is at the top of its stroke, easily sensed by feel to within a fairly small margin of error of crank rotation. What am I missing?

If one really wants to use a dial indicator, what would be wrong with using the compression tester spark plug adapter - it's already hollow, and, with the air fitting attached, provides a good guide for a pushrod.
Click to expand...

Due to the geometry of the crank/rod rotation, finding TDC requires precision. There is very little piston movement around 10* either side of TDC. The difference between TDC and 5* either side is probably only .010" of piston travel.

Precision is necessary for determining your actual TDC for things like validating the flywheel mark, setting cam or ignition timing without a trusted mark, etc. This level of precision is not necessary for rough TDC measurement, like for compression tests.

Larry
 
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Larry DeCamp said:
I am fabricating a plunger for dial indicating accurate TDC to set Pmag timing. It would be simple to sleeve a spark plug and insert a round tip or roller tip push rod to sense the piston. Does anyone know how to get the insulator out of the steel spark plug body ?
Click to expand...

It might be easier to fabricate a piston stop from an 18mm bolt. When using a piston stop, you put a degree wheel on your flywheel (can use sharpies). The piston stop let's you find common points on both sides of TDC (TDC sits directly in the middle of those two points). Even with a dial indicator, this method will give you more accurate results and is the standard for finding TDC.

Larry
 
Thanks Larry

I was nearly ready to type the same thing. Finding the true TDC is pretty important when timing the magnetos. Some engines have no built in marks to verify off of. And I don't trust them either. (Well most of the time!)
 
Got it out !

Thanks to all for the input. The insulator does come out from the plug wire side, however, there are snap rings and undercuts that are designed to secure the ceramic. So, plan a lot of pounding with punces to fracture and remove pieces.

The piston stop bolt is clearly the best approach and the degree scale on the starter can just be masking tape to mark 3 places. The inconvenience is a stable pointer to reference, but I think it is worth it .
 
Thanks for the patient, educational replies, folks, and thanks, Larry, for not objecting to my education taking place on your thread.

I have another curiosity question, if I may?
Once TDC is determined to within a gnats whisker, what is the "real world" experience with respect to the amount of gear lash in the magneto drive train? How does one compensate for or otherwise accommodate that lash to ensure the magneto is indeed accurately timed? Does the act of turning the engine via the prop, as opposed to the crank being turned by pressure on the connecting rods, alter the relationship between true TDC when running and true TDC when being turned by hand (as referenced to the actual magneto drive gears)?

Thanks again for sharing your knowledge.
 
+1 on the use of a piston stop and a degree wheel. And forget tape and sharpies. Locate, then file or engrave a true TDC reference mark once.

To make a piston stop, just weld or braze a short length of 3/16" rod to a spark plug, then trim to appropriate length via trial fit. To make a fancy piston stop, bore a plug body, tap threads through the center, and make up a rod which screws in and out for length.

Turning the engine in the direction of rotation to take out gear lash is standard. Turning the prop by hand is the same as turning with the pistons in operation, although that statement ignores torsional vibration. You can ignore that technical detail too, as there is nothing you can do about it.
 
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Canadian_JOY said:
Thanks for the patient, educational replies, folks, and thanks, Larry, for not objecting to my education taking place on your thread.

I have another curiosity question, if I may?
Once TDC is determined to within a gnats whisker, what is the "real world" experience with respect to the amount of gear lash in the magneto drive train? How does one compensate for or otherwise accommodate that lash to ensure the magneto is indeed accurately timed? Does the act of turning the engine via the prop, as opposed to the crank being turned by pressure on the connecting rods, alter the relationship between true TDC when running and true TDC when being turned by hand (as referenced to the actual magneto drive gears)?

Thanks again for sharing your knowledge.
Click to expand...

I have not done this procedure on my O-320. I suspect my pointer is within a few degrees of accurate and I feel that is close enough for the level of timing these engines use (Much of my timing is set based upon demonstrated performance vs. book numbers and therefore somewhat unaffected by timing mark inaccuracies). One of these days I will properly check it and make a permanent marker on the flywheel.

When building auto engines, you need an accurate TDC reference for things like cam timing and setting initial pushrod length via valve opening reference. As Dan mentioned, lash is taken out in the direction of rotation while taking your reference. While the lash can move around a bit in a running engine, it is most often in the 0 lash state, so that is where you take your reference.

Larry
 
Larry DeCamp said:
Thanks to all for the input. The insulator does come out from the plug wire side, however, there are snap rings and undercuts that are designed to secure the ceramic. So, plan a lot of pounding with punces to fracture and remove pieces.

The piston stop bolt is clearly the best approach and the degree scale on the starter can just be masking tape to mark 3 places. The inconvenience is a stable pointer to reference, but I think it is worth it .
Click to expand...

I made an adapter for my leak-down gauge/compression tester. I used an autolite 386 plug (18mm) and it was very easy to crack and remove the insulator. They are only $2 at the auto parts store; I used them with my EI.

Larry
 
JonJay said:
This product;
https://www.aircraftspruce.com/pages/ep/magnetos_timing/timing_ind.php

Uses a Piston Stop. Their stop is slightly shorter allowing it to make contact on one side of the stroke, then the other. Dividing in half gives you TDC exactly.

I use this and have found it to be a bit more accurate than trying to site down a pin and timing mark. It might be worth the investment for what you are trying to do.
Click to expand...

I should have added my personal philosophy; "why build what you can buy" especially when it is so inexpensive, readily available, and well done.
 
Got one!

Send me a text at five 0 five 7 two zero 19 three five. I've got a box of them from a business I had selling timing disk kits to A&P schools. I'll send you one. I had a local machine shop make me over a hundred of them at a time out of aluminum.
 
14mm solution

Since I have Pmags with adaptor and br8es plug, I did the following:
Thread the bore of the plug with 3/8-24 tap.
Use 3/8-24 bolt in the plug to form the stop.
A 3/8-24 nut will jamb the bolt and secure its position on the spark plug hex.
7:1 compression leaves about 1/2" clearance to the head and plug.
I put a 3/8 delrin tip on the bolt, not necessary ( be gentle )
Rotating the prop both directions to the stop will define two locations.
The middle is true TDC

I have no factual experience to address the concern for gear backlash in two opposite directions.
 
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