While closing up after valve stem seal replacement on an 83 240D manual, I managed to over-torque and break off a bolt holding down the front rocker arm. While I'd certainly be interested in advice about how to remove the stud (photos attached), my main question is about damage to the head: how likely is it that I have damaged the threads on the head by over-torquing the bolt?

Some information about how this happened will probably be relevant. The other bolts went in and torqued to spec without a hitch. I tightened them by hand most of the way and finished with a torque wrench, tightening them to 28 ft lbs. The fourth one turned many times after I put the torque wrench on, enough times that I was nervous and on the verge of backing it out and starting over. And then I heard, not the click of the torque wrench, but a much louder crack.

The most likely scenario, it seems to me, is that I tightened to spec or beyond by accident before I put the torque wrench on, and so, since I was already over the limit I set on the wrench, I never got any feedback from it. If this is true, then I have massively over-torqued the bolt, extracting it will be very difficult, and I fear for the threads in the head.

It is also possible my torque wrench malfunctioned, in which case the bolt is probably still massively over-torqued. Or perhaps the bolt was weak for some reason. I believe these are stretch bolts, but I don't really understand the physics of stretch bolts, so I don't know what circumstances could lead to their becoming brittle or weak (besides the usual things like overheating).

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The reason for not re-using stretch bolts?

The reason is, once you have stretched the bolt once, it never returns to its original size. This weakens the bolt, and if you try to re-use it, you take a risk of snapping the bolt in two, leaving the threaded part in the block, the top in your hand, and a complicated repair required. It's just not worth the risk to try to re-use old bolts.

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96 E300d

As long as the threads in the head are OK don't worry about it. If the heads stripped here is thread repairs for that.

The manual has a specification for the stretch type bolts concerning there re-use. Some are one time use and others you measure and see if you can reuse them.

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84 300D, 82 Volvo 244Gl Diesel

Remove the broken piece with vice grips, install two new bolts and drive on... you'll be fine.

Good luck!!!

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

Chances are, threads in the head are fine. Just get the bolt out and start over. Worst case scenario, you buggered the threads, and need to put in a helicoil or thread insert.

Time for some vice grips, or maybe grinding a groove in the tip of the busted bolt to use a flat headed screwdriver.... If all else fails, just weld a nut to the top.

It's good to see optimistic replies!

Assuming pimpernell is right, and the reason the bolt broke was that it was a re-used and therefore weakened stretch bolt, then it likely broke off under less torque than I had originally assumed. That means it is less likely to have done damage, and hopefully it will be easier to get out.

I'm also glad to hear that vise grips might do the trick. I was sceptical about this, and was waiting to hear some advice before I tried anything. But I guess I'll give that a go first and see if it works.

Four new bolts are in the mail (I'm going to replace the ones that went back in as well in accordance with pimpernell's observation), and I'll see how things go on the weekend.

Put the vise grips on TIGHTLY and place rags around to catch any shavings if they slip. Ideally and probably unnecessary in this case, use two pair of vice grips with the jaws directly opposing each other. The grips and the fastener form a "T" when correct, the broken bolt is the upright and the vice grips are the top/horizontal of the "T".

Apply equal torque to each vise grip, this centers the torque over the fastener and reduces the chance of twisting it off to one side.

Good luck!!!

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

Just re-visited the picture...gonna be difficult to get two pair of vise grips in there. One pair will probably work well.

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

Theoretically, the bolt shouldn't be very difficult to remove. As there is no head on it anymore to hold down any friction on the threads, wouldn't be surprised if most of the broken bolt removal tricks work for this one. You have access to the first 1/2" of the bolt, plus there's not too much stuff in the way around it. Also, the collar around the bolt comes out if you need. It's just a slip fit collar for centering the rocker assembly

Update: The old bolt came out easily. Half a turn with a small vise grip, and then it was loose enough to turn by hand the rest of the way. Not sure what conclusion to draw about that; maybe that the old bolt was stretched to its limit, so it broke very easily.

But now I need some advice on the replacement part. The photo shows what I ordered. There was no photo of the replacement bolt with the description, so I was going on part number alone. I'll cut to the chase: does anyone know if the 17mm bolt shown is a valid replacement for the original stretch bolt, or have I ordered the incorrect part?

How I came to order this part is as follows. I had to consult an MB parts list, which had three numbers for this part (see images). The A112 part is still available, but is cost-prohibitive, especially since I wanted to replace all four. So I ordered the N304 part (which was much cheaper) from Pelican (via Mercedes), and this is what came in the mail.

The new bolt is the same diameter as the old one at its widest parts, so where it screws into the head and where it fits into the alignment groove on the horizontal bar of the rocker assembly. The new bolt is slightly shorter, but that makes sense as the old one was likely stretched to capacity. So it is plausible they could be functionally equivalent.

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Why does the new bolt look so much shorter and like it came from Home D?

Well, I assume the difference in length is owing to the old one being a stretched-to-max stretch bolt. Can't speculate on the difference in appearance. The new ones came directly from MB. The new ones may not be intended to stretch, hence the simplified design? But that's pure, and uninformed, speculation on my part (and assumes they are the correct part, which is what I'm trying to find out).

1 Are the threads correct,
2 can a socket be used to tighten the new bolt i.e.,
3 is there enough shoulder room and
how much does the new bolt protrude through the rocker arm support?

If 1, 2 and 3 are yes and the bolt protrudes about 3/8" or a similar amount to an unbroken old one it should be ok.

Good luck!!!

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

I'll do the protrusion comparison this weekend.

What I'm really looking for is some direct confirmation or disconfirmation that MB used multiple bolt types for this application. I found a photo (attached) in a Pelican tutorial that shows a 17mm hex head like the new bolts have, as opposed to the socket head type that I removed. That tutorial was for a 617 engine, not a 616 (probably an irrelevant difference).

Additionally, I found a note (attached) in the FSM regarding cylinder head bolts, saying that MB had moved from cylindrical shanks to waisted shanks in 79. Perhaps they also did the same with the rocker arm bolts.

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Subscribed .

I was taught that a wasited bolt would be stronger than the straight bolt but is the FSM says differently I'd be inclined to use it .

FWIW, I'd not want to use a bolt that much shorter .

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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