Finding TDC - From The Bottom

[Dan Stokes]

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