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Context
The attached photo is a stud in the roof of my vehicle, to which I wish to attach a roof rack. The roof rack should come with nuts to attach it, but they are missing. I need to find an alternative before the manufacturer can send replacements. [Update] The vehicle is a 2017 Chrysler Pacifica. The studs were used to hold plastic clips in place, which held a trim piece (covering the gutter) in place.

threaded stud

Questiona
What is the name of this type of thread? (with gaps)
How do I match a nut to it?
Why does it have those gaps?

What I already tried
I tried an M6-1.0 nut (close match based on the OD of the stud). I can do a couple of turns with fingers, then it gets too tight. It's easy to put on with a wrench (i.e. very little torque), but when removed the threads show some change of shape. Normally I'd consider any change to the threads to be a sign that the nut is not right, and stop right there. But in this case the threads are strange to start with - it's a design I've never seen before. It occurred to me that the thread may be designed to deform slightly as a sort of built-in loc-tite (with the constraint that they can't be used multiple times). I.e. using the ductility of the threads to prevent the nut shaking lose. Or maybe that's just wishful thinking :)

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  • I’m curious about the thread spacing. When you hold a 6.0x1mm bolt next to the displayed fastener, does the thread pitch line up perfectly?
    – zipzit
    Jun 16, 2021 at 5:25
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    Get a thread pitch gauge and find out exact specification, then you'll know what nut to use. I don't know the name of a thread with gaps like that, not something I've seen too often. I like your thinking about the thread being able to deform due to the gaps but I wouldn't think thats the case, if they wanted nuts to stay in place against vibration etc. then the manufacturer of the roof rack kit could use stop nuts with nylon inserts.
    – NetServOps
    Jun 16, 2021 at 6:47
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    @tom Now that I have a closer look, I wonder if its screw type thread at all. Those lines look horizontal. If they are, then my guess is that the body trim which you removed to uncover those, is pushed into place over that. I assume you didn't have to unscrew any nuts to remove the trim from that roof channel, if you did then obviously you could of just re-used those nuts to secure the roof rack. So, how did you remove the roof channel trim? Did you just pry it off?
    – NetServOps
    Jun 16, 2021 at 7:00
  • Add more info about the specific car. Someone on here meh have the identical car and be able to help.
    – HandyHowie
    Jun 16, 2021 at 7:54
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    Where I have seen these they are often just for holding trim in place. The nut has been made of plastic and the threads cut themselves.
    – HandyHowie
    Jun 16, 2021 at 8:01

2 Answers 2

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I can now partially answer this question.

How do I match a nut to it?
There is no nut that fits this stud with finger strength only. This is likely due to both the shape and the fact it is painted. I managed to get hold of the Chrysler official nut that threads onto this stud. It is an M6-1.0 thread (as confirmed by threading with finger strength only onto a regular M6-1.0 bolt). However a regular M6-1.0 nut cannot thread onto the stud with finger strength only, so it cannot be said that this stud is regular M6-1.0

What is the name of this type of thread? (with gaps)
Unknown. Three theories:

  1. It's partially depitched for retention: https://en.wikipedia.org/wiki/Distorted_thread_locknut
  2. It's just hard to thread because it's painted
  3. The slots in the thread are designed to allow plastic retention clips (for trim) to be better secured (or removed?)

Why does it have those gaps?
See above

I think the only way to get a better answer is to find the engineer who designed / specified it.

Thanks everyone for your help

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  • The Chrysler parts book will have a diagram, showing this part and the others that surround and connect to it. Whatever fits onto the stud will be another part, and have a part number. Jun 16, 2021 at 22:02
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This is a type of self-tapping screw; the gaps are there to allow cut material to displace and to provide increased friction against the fastening coming undone. I’d guess it’s intended to screw into plastic (nylon or ABS) or a soft metal. You could improve a bit using a drilled piece of plastic: possibly you could screw down several layers of thinner material if you don’t have a suitable block to hand.

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