Yes! And in many ways, this is part of what confuses me, though maybe I'm overthinking.

Here's how I landed here. Puzzle #1: why did my original bolt break so easily if there shouldn't be any issue regarding re-use of bolts (I wasn't yet up to 38 NM/28 FTLBS when it broke, and was amazed at how easily the stub spun out). Possible solution to 1: the bolt was a stretch bolt that was stretched to capacity, and so shouldn't have been re-used. But this leads to puzzle #2: why does neither the FSM, nor any tutorial I've read about valve stem seal replacement or rocker arm removal advise caution regarding this? Instead they all say just to put the damn bolts back in and torque to spec. Possible solution to 2: there are two varieties of bolt MB used here, a stretch variety and a non-stretch. The FSM and most tutorials cover the non-stretch. This would also explain the multiple part numbers. And according to the FSM note I attached earlier, it looks like this is just what MB did with head bolts generally, only they made sure to note it, and perhaps didn't do so with the rocker assembly ones?

So what does this analysis overlook?

Just general stuff.

I have had click type torque wrenches that would get a glitch and allow you to over torque. I have set the torque wrong. On Truck Engines I have torqued bolts or nuts to the same torque and had head bolts snap or strip out the bolt of threads.
You never know what the person who worked on the Engine before you did.

Torque wrenches with a dial gauge generally stay in calibration better but you need to handle them carefully and in some positions you have lots of trouble reading the gauge. If you drop one I have seen the pointers come off or the clear covers get damaged.
Click type Torque wrenches have to be of a high quality in order for them to have decent accuracy. But they still get out of order more then other types do.

Occasional you get a Blot that did not get heat treated correctly.

I listened in on a conversation. A guy owned various earth moving equipment. It is not unusual for them to occasionally pop a bolt. He said that the Chinese Bolts of the same grade as US Bolts were cheaper but did not last as long. I mention this because like Torque Wrenches there is a quality issue involved.

That does not answer your question but does show the variations that are involved.

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84 300D, 82 Volvo 244Gl Diesel

So this morning I decided to go out to the 240 and look at the mate to the broken bolt (I had screwed it into its socket to avoid losing it). I think the attached photo is pretty illuminating. I'll leave it here without comment for a bit.

Attached Thumbnails

 

Perhaps the previous installer did not understand how the stretch bolts work?

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

FWIW, over my career I've seen and had more than a few bolts simply give up before they reached the proper torque ~ there's a finite lifespan and it's not always anyone's fault when one lets go .

This replacement bolt gives me the creeps, I'd rather source the correct one be it new or used .


That snapped one is clearly very stretched .

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

From my experience as well, have to agree 101% on that statement.

In the Factory Service Manual both on the CD and in the printed manual there is only a torque on the rocker arm bracket bolts.

Concerning the click type Torque wrenches. I saw a vid on one of the racing teams. He had the Engine on the stand and used a Click Type Torque wrench and went through the tuque sequence clicked on each bolt 3 time. When he was done he re-did all the bolts again clicking 3 times on each bolt and when he was finished he did the same thing one more time.

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84 300D, 82 Volvo 244Gl Diesel