The necked-down capscrews as used in these applications are not torque to yield, rather, they are tightened to a specific pre-load within the elastic range of the material. A torque to yield fastener is tightened past the elastic range and into the plastic (yield) range.

A decided waste of money; the necked-down version has been superseded.

Thank you, I'm happy to be learning things like this. Also thank you for the information about torque vs. shear limits earlier. This was all new to me.

One of the good things about a torque wrench with a dial gauge is that you can use a cheater bar on them because the length of the handle dose not change the toque.
However, the longer handle often puts you too far away to read the dial. You, usually need 2 persons.

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84 300D, 82 Volvo 244Gl Diesel

I too use a click typ, American made torque wrench and click it at least twice .

When I worked in L.A.P.D. Air Support we had a torque wrench testing & adjusting rig, I found it very handy, my old wrench from 1972 or so was almost 1/2 pound off after decades of Commercial Service .

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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

Over approximately 25 years of routinely using Mercedes-Benz FSMs on W123-617s, I have discovered an abundance of instances where said FSMs leave out plenty of relevant information (in my view). Why? I have no idea.

As a matter of personal judgment and choice, I routinely replace relevant bolts on said W123-617s with replacements from M-B. A small price to pay to sleep better at night and stay calmer by day. Your mileage, as the phrase goes, may vary.

Interesting info. My 2 om617 turbos both have the same necked down capscrews that ya broke.

If this kinda thing really bothers ya, drop me a PM. I have a spare head that’s cracked...

I think I'm convinced now that the bolts I bought are the intended replacement part, so I think I'm just going to put those in and see how it goes. But thanks for the offer, and I'll keep it in mind if something goes wrong.

My mechanic is surely not remembering some things correctly, as so many 616 and 617 engines seem to have the capscrews, it seems. In his defense, his son does most of the work anymore, so he probably hasn't taken the rocker arms off a W123 diesel personally in a number of years.

The purpose of necked-down bolts is so the shaft is more elastic, to act more like a spring to maintain a more constant clamping force on something like a head gasket, as parts change dimensions from thermal expansion. Since the camshaft towers have no gasket, that seems unneeded here, but perhaps they used them due to mixing aluminum and steel parts. Most head bolts don't need a neck-down since they are long enough to be elastic. Torque-to-yield bolts are torqued to a value, then marked and turned an exact angle from there (ex. 1/2 turn). That is because the turning torque can decrease once the bolt starts yielding, so if you kept trying to reach a higher torque value, you would just neck down the bolt and break it. This bolt is thus not a torque-to-yield assembly.

Perhaps M-B got reports of snapped bolts so changed to a stronger straight-shaft bolt. Personally, I would have gone to Ace Hardware and looked for a Grade 8 bolt (not sure they have in metric). 28 ft-lbf is nothing, almost 1 hand on a 3/8" socket wrench, so little worry about snapping a quality bolt. Insure you use 2 hands when torquing, keeping one pushing against the head of the torque wrench so you apply a pure couple load and not also a bending load like you would by just pulling on the handle, though unlikely that caused this bolt to snap.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans