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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 :)

tom
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2 Answers2

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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

tom
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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.

Frog
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