Unusual Nut for Headlight Screw

[Alfetta77]

Hi, at the rear of my grilles there is an unusual looking nut that the headlight screw fits into (see large-scale photo). On the other headlight there is a normal-looking nut and washer (bottom right of the other photo). My questions are should both sides have the unusual looking nut?, and if so, where might I get hold of one?

[Alfetta77]

Googled some markings on the nut and found it is a Jack Nut, available from a couple of suppliers, Aerobolt http://aerobolt.com.au/products/threaded-inserts/jack-nut and Concept Fasteners https://www.conceptfasteners.com.au/shop/item/jk-nuts-insert. One less puzzle to solve...

[Alfetta77]

Spoke too soon - bought some M4 Jack Nuts, and the screw doesn't fit  - so off to Supercheap to buy a thread pitch gauge...

[Citroënbender]

That's a disappointment. Can't measure the thread with calipers?

(I'm assuming it's not an interference thread.)

[bonno]

My guess is that it is the fine pitch M4 x 0.5 bolt and you have been supplied the M4 x 0.7 course pitch jack nut/rivet nut. Refer to link for pitch size
https://www.google.com.au/search?sourceid=navclient&ie=UTF-8&rlz=1T4MXGB_enAU573AU573&q=m4+thread+pitch

[Alfetta77]

Thanks for the replies - fine pitch would make sense, will check with a gauge to make sure. I don't have calipers, probably should invest in a set!

[Alfetta77]

Checked with the pitch gauge, seems to be 0.7...and yet the M4x0.7 nut doesn't fit! Will try nuts from another supplier. Any other thoughts / ideas welcome!

[Citroënbender]

Is the screw correct, or has it been swapped for a 5/32"?

[Alfetta77]

It is an original screw, so should be metric? I will try both metric and imperial with the thread checker at Bunnings and see what I come up with.