Snapped a bolt in the trailing arm!

Wavey · 03:00 PM

As part of my complete suspension/brake overhaul, I was reassembling the rear hubs this afternoon. I was tightening the bolts for the rear wheel bearing cover, and the first one snapped off! This is the metal plate that secures the parking brake carrier and the rear wheel bearing into the hub.

I was gradually torquing the 4 bolts down, first hand tight with a 3/8 drive ratchet, then starting with the torque wrench. According to the Bentley manual, the bolts are supposed to be torqued to 34 ft-lb., so I started at 24 and was going to gradually tighten them in the normal cross-sequence. The first one snapped at the same time the wrench clicked, at 24 ft-lb. Luckily I have been able to remove it with an easy-out, and the threads in the trailing arm are OK.

Back to the Bently manual to double check the torque spec. Manual definitely says 34 ft-lb. But then I notice that it also says it's a M10 bolt, but in reality it's an 8mm! WTF! Grrrrr.

Then I go to the front of the manual to the general torque chart, and it says the maximum torque for a Grade 8.8, M8 bolt(which these are) is 24 ft-lb. which is exactly where mine snapped, so at least I successfully tested that spec!

So - what the Hell do I torque these to? Other M8 bolts, like for the rear swaybar brackets, get torqued to 18 ft-lb, but then those probably aren't Grade 8.8, and that is also what the manual calls for on a Grade 6.8 bolt.

I'm thinking maybe 18-20 ft-lb with some blue Loctite. How's that sound? Once the hub is pressed in and the axle assembled, it ain't going anywhere anyway.

john walker's workshop · 10-10-2005, 03:00 PM

16-18 should be sufficient. no loctite needed, just a wavy lock washer.

Wavey · 10-10-2005, 03:01 PM

Thanks, John! Next time try to answer a little faster, OK?

Jim Sims · 10-10-2005, 03:16 PM

Did you have lube on the threads? If so, the M8, class 8.8 tightening torque drops significantly to like 17 ft-lbs. How long since the torque wrench was calibrated?

Wavey · 10-10-2005, 04:08 PM

Nope, no lube - everything clean and dry. The torque wrench is about a year old, never been calibrated since new. But I also have a 3/8 drive in=lb wrench (12 in-lb = 1 ft-lb) so maybe I'll check it against that.

Bad luck followed by good luck: I just found 4 new exact match bolts, so I'm good to go tonight. 2 of the other 3 bolts were already stretching at less than 24 ft-lb, so they'll all get replaced.

wswilburn · 10-10-2005, 06:20 PM

Bentley has this wrong. There are just a few incorrect torque settings in Bentley. I seem to remember something associated with the engine mounts is also wrong (mounts to transmission crossmember I think). Would be good to compile a list of these errors.

Wavey · 10-11-2005, 02:53 AM

Great idea, and done:

The Official Bentley Errors Thread

jpahemi · 10-11-2005, 03:52 AM

The standard hex head bolt grade on the pre '89 911's is Grade 8.8; areas such as the trailing arm and spring plate are 12.9.
10.9 usually holds the aluminum cross member in the front. For the most part, M8 bolts are usually 8.8, with the exception of the CV bolts (12.9).
j.p.