Problem with stripped threads on spring plate

[D911SC]

I have an issue with stripped threads on the nut/bolt assembly that attaches the spring plate to the trailing arm. This is an M12 x 1.5 x 35mm bolt, 2 thick washers, a lock washer and an M12 nut.

I set the torque wrench to 120Nm as per my Bentley manual and started to tighten. Sure enough it never really got super tight and before I knew it the thread was stripped. The nut and bolt had been cleaned and replated plus I had used antiseize.

I didn't have a spare nut and bolt so I went to my local mechanic (Porsche specialist) and bought new ones. So I tried again and exactly the same thing happened. Now I just felt stupid!

I then thought that perhaps it was the torque wrench, even though I had tightened up an M14 at 125Nm with no issue. I grabbed another nut and bolt and put the nut in a vice then tightened the bolt to 120Nm - no problem at all. I then put the same nut and bolt on the car and bingo - stripped again??

I have no load on the spring plate/trailing arm. The rear shock is disconnected and the bolt slides freely on and out of the hole. When tightening I can see the thick washers slightly deforming as I tighten but it never really gets tight before it strips.

Any ideas what this could be. I'm thinking I need to get some seriously heavy duty nuts and bolts but I can't understand why this is happening.

By the way I tried tightening the bolt (torque wrench on the bolt) and also the nut but same result.

I've already done quite a few bolts with the torque wrench and this is first issue I've had like this.

Any suggestions?

[D911SC]

Is the torque setting right? I double checked the book but researching tolerances it appears that 120 is too high for an M12.

[R_Builder]

The 82/83 Tech Spec Book states the spring plate-to-control arm bolts should be torqued to 95 Nm (70 ft-lbs) and their material grade is to be 10.9. Verify you are not using 8.8's (although an M12 8.8 shouldn't strip out at 70 ft-lbs) and try 95 Nm and let us know if that works out better...

[KTL]

The Bentley has been known to provide inaccurate torque specs.

I'm surprised you deformed the washers, as these are fairly thick compared to a standard M12 washer. Also, the original hardware is bolt, thick washers and deformed thread all metal lock nut. If you can find some of the all metal nuts, then you can ditch the annoying lock washers.

The M12 x 1/5 is a bit less common at local sources, so in a pinch you could substitute 1.75 pitch and new nuts. My original hardware was a bit aged and the locknuts were not locking all that much, so I got some new 1.75 pitch bolts and nuts from a local industrial supply house.

[D911SC]

Thanks Kevin. When I was picking up bolts at the local Porsche shop they pulled out their workshop manual and it showed lock washers and normal nuts. Either way I guess both will work.

I went to a bolt supplier and all they had was normal zinc (ie. not yellow) in 10.9 with 1.5 pitch threads. They had 8.8's in various pitch's so I got an assortment. I set the torque wrench to 95 and used the 10.9's. They tightened up no problem at all. I also got some new washers and they didn't deform at all in the spring plate recess.

I still can't explain it but at least I have a solution - although I'm sure my mechanic will frown at the use of non yellow nuts and bolts

Thanks for your comments Kevin and Doug.

[KTL]

That's weird RE the lock washers? Maybe they were looking at the nuts for the toe & camber alignment fastenings? They're sort of jumbled together in the parts diagrams and easy to confuse. The PET pdfs and my paper manual (74-89 Porsche Parts Reference) show only locknuts. No big deal like you said. Just find it funny that the dealer doesn't get you the right info. Probably don't see many of the older cars anymore?

I checked my notes from the Carrera spec book and 90 ft-lb is the spec. 90 is reasonable for an M12 10.9 fastener according to this doc

http://www.metricmcc.com/catalog/Ch10/10-1027.pdf

Sounds like you have it all worked out. Good job.