Mercedes Bushwhacking

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When a one day job becomes a two day job because a part is incorrectly specified or otherwise wrong, you can be sure that you are having an excellent time. When the new self-locking nut intended to retain one end of my driver's side front suspension carrier sheared away from its magical captive shim during yesterday's automotive repair attempt, I knew that I was in for a really good time.

My '82 300SD needed a driver's side braking support rod bushing badly (clunk-clunk-clunk over the bumps we went) so I ordered up a Meyle-branded replacement as I've been very happy with their parts up until now. It was a good-looking part which came with all the hardware as is generally necessary for almost any kind of parts replacement on the W126, as practically everything is secured with a lock nut (the squished kind) and you are instructed to replace bolts and/or nuts each time the part is removed. So I had got through the removal portion of "33-600 R and R braking support link ball joint carrier" and all of "33-615 R and R braking support link ball joint" and was well on my way to finishing the replacement portion of the first job when everything went wrong.

The Mercedes has a sort of multilink suspension in the front and a semi-trailing in the rear, which we don't need to talk about now (unlike when I replaced my rear springs.) The front suspension is quite a bit more interesting, anyway. Everything is cast and made with efficient, swooping shapes. A braking support rod runs towards the rear and attaches via a ball joint in a bushing to a cast aluminum carrier which attaches to the unibody via two big rubber bushings. The carrier is retained by two fairly long, fat bolts; I'd say completely at a guess that they're M10x1.25 or 1.5mm thread pitch and about 8cm long. And these bolts run into what seems like a really nifty fastener, a square lock nut with beveled corners made captive to a metal shim that snaps into place in slots in the unibody.

This fastener is in fact really nifty if it's made well enough to take the substantial fastening torque of 80 N-m, or 59 lb-ft. Unfortunately, the ones that Meyle included in my kit (and even stamped their name on) aren't. Long before I got there the nut seized the bolt and disassociated itself from the aforementioned shim. Since the nut is square, but not double-square, there was no conceivable way to hold it. I knew I would have to drill it out.

This morning I went to Kelseyville Auto Salvage and got the bolts and their backup hardware. I also picked up some Mercedes lug wrenches and, a real score, an official 1982 300SD Owner's Manual reproduction. New hardware was necessary because in the case where you cannot remove a nut and you only have side access, you remove it by drilling through the side of the nut and then splitting it. With more access I could have used a nut splitter, which goes over the top of the nut and takes an impact tool to operate, and I've got a compressor and impact gun too, but there's just no way to get my nut splitter onto that nut. A nut splitter sometimes preserves the bolt, but drilling almost always damages threads and you're best off replacing the hardware. So in I went, and drilled that sucker out, and then took the drilling sledge and a nice little chisel to the side of it. The chiseling did stamp some thread marks into the reinforced section of the subframe where the bolt passes through, but no serious harm was done.

Then I did it all again, but being compulsively careful about keeping the threads and the nut clean, and it happened again. This time, however, I was much better prepared for the job (having just done it once) and so I removed the second nut much more quickly even though it was actually further onto the end of the bolt. This was a necessary step if I was to bad-mouth Meyle in the future for producing incorrectly specified parts. Consider this done. You're not supposed to re-use the backup hardware, but all I had left was the back-up hardware I had pulled, so I used some Super Glue brand red thread locker. The red stuff is the strong stuff, so it's the right choice for something critical like the suspension. It came with my latest turbo rebuild parts from Mercedes Source, so I had it on hand already. Normally I buy Permatex, not because I care, but because it's in the LAP (local auto parts) store.

I want to address this practice for just a moment. If the manufacturer tells you not to reuse the hardware, there is probably a reason. The usual one is the use of cheap self-locking nuts, which are basically normal nuts (with some minor changes) that have been squashed slightly on one end. When you run a bolt into them they deform from their new, arguably deformed shape, and pinch the bolt. This is in fact the type of nut I had a problem with here. In some cases you may be able to substitute alternate hardware. For example, in this case it might have been possible to feed a self-locking nut into the hole and hold it with a side-offset box end wrench, like you'll typically see on long wrenches with double box ends. But I chose to reuse the hardware because it still fit tight, and I was still able to torque it to factory specs without it feeling like I had damaged something, run out of thread, et cetera. Use your own best judgement and if there is any doubt, do not bet your life on old hardware.

You are instructed to push the control arm forward to be able to mount the carrier, but I found that it was possible to simply run the bushing's thread rod all the way into the braking support rod (caster adjustment) and then install the rear bolt. Afterwards, you can pivot the carrier around the rear bolt and slip the front bushing over the projection on the unibody, then run in the bolt. After this job, naturally, you are supposed to have the alignment done, and I might do that. Unfortunately, while I was down there I noticed that the lower control arm bushing is going, so it might be a good idea to address that first.

All in all, this is theoretically a fairly simple job requiring only 13 and 19mm wrenches, a socket wrench, 17 and 19mm sockets, an 8mm hex (the manual says 10mm) driver you can turn with a socket wrench, and a torque wrench. Remember (if necessary) that the conversion from N-m to lb-ft is about .75, so if you wanted 100 Nm you'd want about 75 ft-lb. My clickers are marked in N-m and lb-ft and my beams are marked in kg-m or something daft and in lb-ft. But if you get the Meyle kit, you may well consider reusing the old hardware and using thread locker, especially if you don't have a good set (in 1/16" increments or smaller) of titanium or cobalt drill bits!

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