Finding TDC - From The Bottom - PeachParts Mercedes-Benz Forum

Dan Stokes · #1 08-26-2015, 06:01 PM

My timing indications on the OM617 were coming out inconsistently. Given that I had "issues" reinstalling the dampner I knew it was at least possible that the dampner was off a tad so the marks would be off. You can't readily come thru the head as the injector and glow plug both enter the pre-chamber w/o direct access to the combustion chamber.

The good news is that access to the lower oil pan is pretty good as long as I drop the steering linkage (not sure of access in the Mercedes chassis). HMMMmmmmm - can you measure it from the bottom? The short answer is "YES" and it isn't too difficult once you work out the method. Here's the pics:

The timing mark TDC as I found it.

Pan off and dial indicator setup. There was a little fab work required as the upper pan is aluminum and I wouldn't trust the mag base anyhow. You might not notice that 1/2 of the oil pump is also missing as I needed the access to #1 rod.

Close up showing the dial indicator extension (6" from ENCO) right on the bottom of the rod. Dead center of the bottom of the rod has minimal side to side movement at approximate TDC.

At TDC. The Dial indicator gets as long as it will get - move it either way and the measurement gets shorter. Just to verify this I went 0.020" either way (of course, the indicator goes the same direction) and marked the dampner. TDC would be the center between those 2 marks and in fact they came out exactly where the indicator showed the longest reading (zero as I set it).

Here's my 2 marks. Turns out that TDC is slightly different from the factory mark, like a degree or so. My actual TDC is JUST to the right side of the pin (right side of the truck, left as you look at it) (see first photo).

This whole deal took stupidly longer than it should have (though we had a 2 week vacation in the middle of it) but I could do the second one in an hour or so. I'm pretty sure that this technique is wildly accurate and could be copied on pretty much any engine. I can't stress how exact the longest measurement is (I set it to zero) and how little movement it took to take the reading off zero. I expected the rod bearing clearance to enter into it but it doesn't seem to have much effect.

Dan

leathermang · #2 08-26-2015, 06:34 PM

I hate to throw water on your enthusiasm .... but that is not an accurate way to measure TDC . There are discussions in the archives on this....and accurate measurement involves INdirect measurement of the crank position by measuring the piston at equal distances from the top of its movement.... and marking the crank at both the upward movement...and the downward movement....and splitting the distance between the two marks represented on the crankshaft pulley or damper.
There are several degrees of crank rotation where there is virtually no up and down movement of crank both at TDC and BDC.
The most accurate determination involves having the crank throw at 90 degrees before and after TDC.... found by marking equal distances before and after movement past the top of the cylinder by the piston...
That is because for each degree of movement of the crank that is the point at which the maximum movement of the piston happens.... so the combination of a micrometer on the piston at that point... and matching after passing the top of the piston movement.... is physically the most accurate measurement possible.
I do not know where you got the idea that the top of the piston was not accessible via removing of the precombustion chamber..... that is just not true.

http://mgaguru.com/mgtech/engine/cs111c.htm

http://www.shoptalkforums.com/viewtopic.php?t=123659

""You see, one problem is that the piston stays virtually motionless while the crank turns over the TDC position. The crank keeps turning, but the piston doesn't move until the crank starts pulling it back down. This brief moment of piston wait, or "dwell", or "death", can last for several degrees of crank rotation."""

Dan Stokes · #3 08-26-2015, 08:40 PM

Let me try this again -

I didn't want to pull the precombustion chamber. Mine seal and I prefer to leave them that way.

Now as far as accuracy, I'll pit my method against any other. The dwell period does exist and I accounted for that by measuring either side of my zero movement point. If you actually did this procedure you would find that that dwell period is VERY small - about +/- 0.001 inch or so. It's not hard to make a darn good estimate of the center of that movement.

The method you describe no doubt would yield a VERY accurate measurement but mine is plenty close enough for any practical usage. I've built many a race engine with no more accurate a determination than this - and so do almost all professional builders. The common method is to use a bridge and a dial indicator to find the point at which the piston is at it's highest in the bore and label that as TDC. If you look on Jegs or Summit you'll find the bridges listed as an engine building tool.

To measure TDC on an assembled Otto-cycle engine you pull #1 plug and insert a stop. Then you turn the engine each way until it hits the stop and measure half way from the marks to find TDC. This is approximately the method you are describing. Practical experience shows that the bridge and the stop method yield comparable results.

One of the issues I've seen on this forum is that there are great guys on here but many seem to lack practical experience though they have lots of book smarts. This is one of those occasions. In reality, if TDC is within +/- 1/2 a degree or so it'll work just fine.

Dan

leathermang · #4 08-26-2015, 08:49 PM

You did not read the urls I posted.
I gained my knowledge from back when vibration dampers were held by rubber and on small engines which had no marks on the crank end.

."""" I'm pretty sure that this technique is wildly accurate and could be copied on pretty much any engine. """"--- Dan Stokes

I am just saying it is not ' wildly accurate'.... because of the definition of the physics involved.

And for the record... you did not have to pull your well sealed injectors or precombustion chambers...... as this top side method can , on our diesel engines, use a loose valve sitting on the top of the piston.

Those piston stop tools are not useful on OUR engines because the chain design does NOT allow our engines to be turned backwards to come up on the other side to that stop...

Diesel911 · #5 08-26-2015, 09:45 PM

I tried to attach the whole pdf but either it is too large or my dial up internet is not up to it.

I posted a pic of the last steps of finding TDC. What I get from what I read is that the Manual is saying TDC is when the Dial Indicator first reaches TDC. It is not asking you to average anything. Step #17

Also the Manual has you getting the Piston up near TDC and removing the Rocker Arms, Valve Nuts, Spring Retainer and the Springs and letting the Valve go down rest on top of the Piston.

If you are affraid that the Valve may go too far if you can attach some fat O-ring around the Valve Stem so it cannot drop too far. One would hope there would be at least enough tension from the Valve Stem Seals to allow you to attache the O-ring.

The Dial Indicator Plunger goes on top of the Valve Stem.

It is 03-345 in the Engine Section of the CD Manual W123.

Diesel911 · #6 08-26-2015, 10:32 PM

The W126 has the same Engine.
Go to section O3 Crankshaft and then find section 345 it will say "Checking and correcting adjusting of TDC Transmitter

Mercedes-Benz 126 Service Manual Library

mannys9130 · #7 08-26-2015, 11:53 PM

Good info to know. Even your degree marks being just a degree off is not bad at all. The chains stretched 2 degrees right off the bat within 10k miles, and injection time +/- a couple degrees won't destroy an engine.

Good work.