Felix

A friend has just had a rather disconcerting failure on a Bilstein HD strut on the front of his C4S - the piston or shaft broke at the threaded section underneath the nut. Thankfully it happened at low speed and there was no collateral damage other than to his underpants when he discovered the source of the noise and thought about what *could* have happened. One interesting point - the depth of the hex fitting in the top of the piston is 7.5mm on the broken strut and 5.5mm on the other - this could well be significant. Anyone with HDs care to measure theirs? The struts are about 2 years old and it's a fair weather summer car with no track use. Although its fairly common to have internal hex fittings on the tops of strut pistons it does appear to create a weak spot if the hex is deep enough so that it extends more than half way to the base of the nut.

Stealth 993

That is crazy, I have never seen, or known of this to happen.

Call Bilstein, they should send you a new shock.

Felix

Bilstein has been contacted - a reply is awaited.

993Porsche

Yeah, I'm also very interested in their reply.


Ken

AOW162435

Do you know if the nut was installed with an impact gun...?


Andreas

Felix

No idea - it was an experienced and long-established independent shop who specialises in Porsches.

VNTGSPD

Looks like it was over-torqued. That part of the shaft is not heavily stressed during use. Unfortunately can't really verify this without some metallurgical testing though.

Look forward to hearing the results.

Felix

Why do you say that?

VNTGSPD

The failure in the threads is near the bottom of the nut where the "stretching" of the shaft would be most extreme if over-torqued. This area does not take the load from "bounce" or "bottoming out" and does not take the sheer load from cornering forces. It's only significant load is during "rebound" which in my opinion would never be strong enough to "pop" the top of the shaft off. The wheel would likely come off the ground first.

It certainly COULD be a manufacturing defect, but most fastener failures in my experience have to do with improper installation.

Not sure what to make of the dirt/grease in the pictures as it appears to be smeared into the area where the break should be clean.

I'd look at the piece of shaft left in the nut as well. This should turn relatively easily inside the nut if the remaining threads are not too damaged. If it feels "welded" in, then the threads might have galled upon installation. This could cause a torque spike into the shaft. This is a common problem with nylon insert locknuts that are installed with impact or high speed drivers. That being said, I think the shaft turns freely inside the bore, largely eliminating this possibility.

Steve Weiner-Rennsport Systems

While I cannot comment on this particular failure unless I could see it for myself, I've seen this happen and IMHO, its been due to the use of an impact gun at installation.

I prefer using a standard (non-nylock) nut and using a dab of blue loctite to secure it and run it down with an air ratchet.

VNTGSPD

I completely agree with Steve (duh!). While I don't have the experience of the shop owners/mechanics on this board, I am a fastener guy by trade and specialize in self-locking. I see similar issues in various applications all the time.

Good luck in getting it resolved.

chris walrod

Which is presumably why Moton doesnt use nylock nuts.

EckFe1

Holy crap, I had to run out and go check on my H&R kit. The hex depth of the strut shaft, measured with a caliper, from top of thread is 8mm with the hole to continue to a max. depth of 12mm on both sides. The nut, no nylock, is well beyond the hole, I'd say about 1/4" deeper then where the hole ends. For it to break the shaft, it would have to shear the full diameter of the shaft, not only the sidewall between hole and OD.
I believe the shock wasn't installed properly. Maybe some of the spacers that go below the tower were the wrong thickness?
On your setup, somehow the shaft is too short or something got really messed up during the install. You'll have to take off the other side too and start to investigate. There's no way that Bilstein would have build it that way.
I just came home from a drive that was way over 120mph for most of the way of my 1.5 hour drive and I couldn't even imagine having a failure like that at that speed.
Keep us posted and good luck with the repair.
Ed

berni29

Hi

If you manufacture enough of those things eventually one will have a defect. That could be one of them.

If I were Bilstein I would send you a new or remanufactured pair.

I hope that it is resolved to your satisfaction.

Berni

Steve Weiner-Rennsport Systems

Thats correct,....