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Discussion Starter #1 (Edited)
I ordered some 8mm OEM lock nuts from Europa Parts (N90894601) for a downpipe replacement. The nuts are copper (or copper coated) and easily go on and off the OEM 8mm bolt which appears to be 10.9 zinc. Even when I tightened to 30ft-lbs as a test once I loosened the nuts they were loose, the threads did not deform.

I am concerned when I use the nuts to replace the cat-downpipe connection that they will vibrate loose. I asked EuropaParts and they said nobody has reported a problem and to e-mail a picture to their support line. My question is aren't the threads of a lock nut supposed to be deformed or deform when tightened? Otherwise how the hell are they locking?

 

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Can't say, but if they are copper, you should be able to deform them enough with Vicegrips to increase the thread friction.
 

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Normally they are crimped on the end or deformed right at the thread entry / exit of the hole.

If there is no visual of that, the threads might be manufactured to an imperfect thread, which when mated to the bolt, will cause a locking effect due to the slight mismatch in thread pitch.
If the nut feels like it gets tight right away when turning it onto the bolt, then it's an interference thread fit.

We do that kind of stuff all the time where I'm at.
 

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All the new flange nuts I used when putting my exhaust manifolds back on my truck were all tight at the end to the point that you could only turn it about 4 threads by hand then it would require a tool.

The nuts that came with the various other exhaust components were the same way.
 

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These aren't they roll loose. I wonder if they sent me "Genuine OEM". Have you ever done an exhaust flange install, do you recall the nuts being lock nuts?
All of the exhaust work that I have done on Passat's, the copper nuts on the exhaust manifold and down pipe connector on the manifold have been some type of crushed nut that you'll get maybe couple of turns by hand and then after that a wrench is required.

These here from ECS Tuning are actually crushed from the outside at 120 degrees apart (3 places equally) and the thread is undersized at the 3 points. When turned onto the stud they are pretty tight.

Now there is a chance you might have a bad run of exhaust nuts. The manufacturer of those things chunk them out by thousands per shift. It's pretty easy to have a few non-conforming ones slip through. I'm in manufacturing so I know these things.

 

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Discussion Starter #7
Now there is a chance you might have a bad run of exhaust nuts.
I think that is probably what happened. All the 8mm OEM "lock" nuts I got from Europa were like that, not just one. I bought some N90894601 nuts directly from VW and they are slightly oval at the top due to what appears to be a crimp and so they can't be turned by hand past that point. However I used a torque wrench to turn them and they need almost no torque to turn because even at the lowest setting of 20in-lbs I didn't get a click. Better than what I had before at least.

OTOH the 10mm OEM lock nuts I got from Europa take seemingly too much torque to turn, like at least 20ft-lbs. I didn't have a torque wrench for testing it but it took a lot of force and it made some metal shavings I assume from the threads. I took the nut off and the copper coating was gone from the threads.
 

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Discussion Starter #8
An update on this, I went to a VW dealership and bought the same 8mm part and they are locking, but in a minimal way. Maybe at most a few pounds of torque required to turn. The tip is crimped and it looks slightly oval. Anyway they are different from the Europa ones.

These here from ECS Tuning are actually crushed from the outside at 120 degrees apart (3 places equally) and the thread is undersized at the 3 points. When turned onto the stud they are pretty tight.
What's the part # on that? If I do any more exhaust work I'd like something with some more resistance to it, my exhaust vibrates like hell.
 

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