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Stripped Shock Mount

Well, the threads in the left trailing arm are stripped such that I cannot torque the shock absorber M14x1.5 bolt. My intention is to Heli-coil it, as I have lots of experience with these.

Anyone got any alternate suggestions, or successful repair stories?
Thanks!
Pat

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Old 10-12-2005, 06:05 AM
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Well. since the alternate solutions never arrived, I said screw it anfd put in a Heli-coil.

In the photo, you can see a hex shaped bit of steel. Does anyone know if this is a Porsche insert into the aluminum trailing arm? Can it be replaced? I only had a 21mm length Heli-coil, so I went with that, but it needs to be longer. It did work, however. I do have a bit of concern about the long term durability.
Pat













Battery died in the camera at this point. Sorry.
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Old 10-14-2005, 09:52 AM
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I dunno - but it is a good bet that you want the strongest possible "re-thread."

There are also items called Time-Certs - you've probably heard of them.

And there are some other things too but I don't recall their name.

I would go to a real good fastener shop and see what they have (in metric) and ask the best guy there what is the strongest - tell him the loads are primarily in shear.

Also, keep on bumping this thread and maybe Jim Sims will post for you. There might be some other engineers with a lot of fastener experience on the bbs too.
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Old 10-14-2005, 01:01 PM
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i want one of those...
 
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Quote:
Originally posted by patkeefe
In the photo, you can see a hex shaped bit of steel. Does anyone know if this is a Porsche insert into the aluminum trailing arm? Can it be replaced?
I'm pretty sure that steel nut in the alum trailing arm was put there by Porsche. Not sure if it can be replaced (probably pressed in) but it was present on the two sets of arms I had.
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Old 10-14-2005, 01:22 PM
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Unless you're using coil-overs, the only stresses on that bolt come from the valving of the shock, which is to say not much.

A well executed helicoil and some medium strength threadlocker should work just fine.
Old 10-14-2005, 01:28 PM
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Pat,
You did exactly what I would have done...Your fix should serve you well into the future. Nice work by the way.
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Old 10-14-2005, 03:25 PM
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Thank you all...back in my machinst days, I probably put in about a thousand various repair thread devices. This particular installation only gave me 21mm thread, and the bolt threads are probably 40mm. But I did get the 90+ lb-ft of torque on it. I'll look around for a longer heli-coil down the road.
Pat
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Old 10-14-2005, 03:26 PM
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Randy Blaylock has a great first name, but he is forgetting about what happens when you hit a big pothole or bump and the shock rams into the bump stop. At that point, forces will greatly increase. Maybe a helicoil is plenty strong - I dunno - BUT you have to design for a worst case scenario... and you are designing.
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Old 10-14-2005, 04:04 PM
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Point taken.....
Old 10-14-2005, 05:06 PM
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I think you will be fine. Even bottomming out the shock, I wonder how much force there will be on the threads? Most of the force is certainly in shear as Randy Webb first pointed out.

Cheers
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Old 10-14-2005, 05:11 PM
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Yeah, this is my point of concern. I now have only 21mm grip length, and not 40+mm grip. Intuitively, I could introduce additional rotational loading on the shorter heli-coil which is not as great in either the stock set-up or a full thread repair. This does not, however, introduce additional shear load into the bolt; that would remain the same. Porsche evidently considered this during one of their late nights at the brauhaus, hence the large diameter long thread bolt.

I suppose I could do a free body diagram on the bolt and determine the actual forces at work. But, in reality, I'd really have to bottom the car hard to reach the bump stop in compression. Those shock bolts seem pretty durable.
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Old 10-14-2005, 05:19 PM
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The other way to think about it is: what is the worst thing that could happen if it breaks? -- probably not a real catastrophe...

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Old 10-14-2005, 08:13 PM
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