*Chain stretch via dial indicator
 

Who’s used this technique on an om61x?

I’m at 215k.
Via crank method it’s always shown 2 degrees.
Last I checked, it was 3.5.
Thinking it might be woodruff time.




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Others with more skills and experience than I are the ones to listen to, but I would not change my camshaft and injection pump timing without doing the test as recommended by the factory manual. It's not that hard. Then, having accurate numbers, one can judge whether a new woodruff, a new chain, or nothing at all. Seat of the pants stuff makes me nervous.

This is from my notes. Note I have never done it so re-check on the notes.

5 Degrees Offset Woodruff Key 1.3mm = 10 deg. at crank Offset woodruff key for correction of cam timing.
3 Degrees Offset Woodruff Key 0.9mm = 6.5deg. at crank
2 Degrees Offset Woodruff Key 0.7mm = 4 deg. at crank

It won't make any noticeable running difference with so little stretch

Yeah but that’s off the crank. From what I understand my reading could be off by as many as 5 degrees. This is why I’m curious about checking chain stretch via dial method.

Exactly. The dial method is the method recommended via FSM, ergo my query.

I have, a few times on at least two OM616s. It is the best way to know cam timing accurately. You need a dial indicator that is scaled right for the measurement, and a way to hold it in place.

I used a magnetic base with jointed arm for holding the dial indicator in place. I bought a set of extensions for the rod to keep the tool above the rocker and cam, which provided a straight shot down to the valve spring retainer. It helps to use a longer tool for rotating the engine, so you have greater control and are less likely to overshoot the 2mm depth.

Here's what I'm looking for.

Did you end up using any woodruff keys to correct?

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I did, in the '74's engine. I installed a 0.7mm offset key as mentioned above in post #3. That restored cam timing to spec but I didn't pick up any noticeable power, that's for sure!

While using a dial indicator to measure the movement of a valve provides an admirable degree of accuracy, the reliance upon the production marks from the factory to determine the position of the crankshaft tends to negate the usefulness of the indicator's accuracy.

Who is to say that the pointer is correctly located in the front of the chain case? Who is to say that the circumference of the balance disc or torsional dampner is correctly marked for TDC or the number of degrees before or after? If TDC has not been verified for all of the specific components (crankshaft, balance disc/dampner, pointer) then only one part of the reference, the valve lift, is accurately known. Crankshaft position is not accurately known.

Ah, a post-structuralist... ;)

Have you encountered many om61xs with factory defective crank pointers?

Do you have a better method?

Oh, Poopy Poop!

As for the accuracy of the scale, I trust German engineering and manufacturing skills. Plus, on these engines those whacky Germans provided a way to adjust the pointer to match actual, for real TDC:

- Pull #1 injector and pre-chamber;
- Insert wood dowel rod into the hole 'til it rests on the top of the piston;
- Rotate engine at crank on compression stroke for #1 until the dowel indicates true TDC for #1;
- Check the pointer on the front of the engine for accuracy against the scale. If found to be inaccurate loosen its retaining bolt and adjust the pointer to indicate TDC; and
- Problem(s) solved.

:D

Was Socrates a post-structuralist?

See below.

The concept of the above described process is both correct and necessary. For the dowel substitute a dial indicator, note the crank position with the piston ~.100-.125" both before and after the presumed TDC, and split the difference. There will be a dead area of piston movement + 2.5-3 degrees at TDC where dowel movement will not be detected, hence the need for bracketing the piston position via the indicator.

Even better! Use the dial indicator with rod extensions inserted through the pre-chamber hole to establish TDC. That's a great idea and more sure than observing movement of the dowel rod. I gladly update my process as outlined above to include the use of dial indicator instead of a dowel rod.

I'm sure the factory guys got the pointer set right to begin with. Could it have been moved from the correct position at some point in time? Yes, it is possible. You raise a valid point, Frank. If in doubt check and re-align if needed.

[QUOTE=Frank Reiner;4127807]Was Socrates a post-structuralist?

The Socratic response would have included both point and counterpoint.
Looks like we got there in the end.

Thank you gents, I appreciate the techniques.
Lately I’ve seen more discussions on rolling a new chain, than those using offset keys. Could be a function of age or just a dying art. I have been curious all the same.

Thanks again

Actually if you have a long enough extension you can do as you said and hoop up a dial indicator to get TDC if you want to check the pointer.

I think it was Army/Stretch that mentioned you can bring the Piston up close to TDC and remove the upper Valve Spring Retainer and let the vlave go down and contact the piston. Arrange the dial indicator so that the extension is on the top of the valve stem and slowly and carefully continue the rotation in the proper direction till TDC. Then look down and see if what is lined up with the pointer.

Concerning the match marks on the cam Tower and the back of the timing gear. The angle that you are viewing them at can throw off your reading.

Concerning Engine Match Marks in general they are used all over the world on any manner of Engines and have proved sufficient for practical purposes. And these Engines have been around long enough for some issue to have previously shown up and be known concerning the Engine Match Marks.

[QUOTE=Shern;4127843]I think a lot of the trepidation over when to replace a timing chain is due to some makes of Cars needed the Timing Chains replaced after say 150 thousand miles changed due to the Nylon gear teeth on the larger gear and the use of no timing chain tensioner (when I replaced that on my Chevy 4.3 liter engine both replacment gears had steel teeth).
Next would be the Mechanics either uneducated in Mercedes 617.952s or looking for work to scare you and trying to sell you a timing chain replacement job.

It seems pretty well know now that on the 617.952s it is the Timer Bushing that is the most frequent cause of the Vacuum Pump failures that snap timing chains and damage engines. Next in line would be some issue with the Vacuum Pump.

The other thing but not specific to the timing chain is if that little snap ring on the Oil Pump Chain tensioner wears out and the tensioner shoe move around and caused binding on the Chain. Or if the spring broke.

I guess next would be an issue with the The Timing Chain Tensioner rail or the tensioner.

Notice that the above has nothing to do with the IWIS/jWIS timing chain itself.

911 to the rescue, eh?
Thank you for that. I’ve always felt the cam/dial issue was a little over blown.
Reminds me a bit of speeding ticket litigation in the 90s. The argument being the position of driver’s head caused misread of speedo dial. I’d love to see a comparison of the dial gauge method versus cam/balancer marks.

The timer bushing issue pertains to the 616 as well I’m assuming?

Having timed at least 200 sets of camshafts for maximum power on a dyno there are a few observations:

1. 3 degrees is not going to be noticed with a conventional street cam in a diesel. You would need a very high performance cam for it to make more than a 2% difference
2. The dial indicator method is not all that accurate. You can easily be off 2-3 degrees when comparing to hard stop index methods used by race teams and OEM production.

The decision is always up to the owner, but don't expect much.