So I got a cheapo 2½ inch Aston lookalike petrol cap off ebay.
Being a logical guy I figured the connection would be a 2½" pipe thread. I stopped at the local industrial supply shop to pick up a 2½" threaded weld nipple but nope it was too large to fit the female thread of the petrol cap so I left empty handed.

When I got home I started looking at the threads more closely at it seems they are slightly conical? at the bottom the diameter measures 68,4 m but at the top (entry thread) its closer to 69 mm. I measured the distance between three thread peaks and got 4,8 mm which leads me to think that it would be 16 threads per inch ? I googled some BSP charts and none of the dimensions really match up? The 2,5" seems to big and the 2,25" seems to small.

If anyone has more experience with inch threads I would greatly appreciate some help.

Threads of a form can be cut to suit any diameter, a car manufacturer may opt for an odd diameter to suit need or aesthetic. If you can identify the number of teeth and the tooth form a friendly lathe owner would be able to cut the mating thread to suit.

Hth

Hi

I think that's Metric just to be awkward.. M70 x 1.5?

Try giving Think Automotive a ring: https://www.thinkauto.com/caps.htm

Also see: https://irp-cdn.multiscreensite.com/e112be6e/files/uploaded/cap2.pdf

https://www.merlinmotorsport.co.uk/s/fuel-system/fuel-caps-and-fuel-filler-hose/flanges-aston-monza

Thanks
Mike

[Edited on 17/2/22 by Mike Wood]

[Edited on 17/2/22 by Mike Wood]

that seems to match 2 3/4" x 16tpi UN exactly, matching this:

https://www.carbuilder.com/uk/weld-in-2-34-16tpi-threaded-filler-neck?gclid=CjwKCAiAgbiQBhAHEiwAuQ6Bkq247B1QLhWLNHbcSho-qR-TeNHPkoKpTDAj8MRY5uNQ-mkBBb J30RoCn18QAvD_BwE

Sounds about right, pipe threads are related to the bore of the pipe not the od

Thank you everyone, I am 3D printing a test piece with the 2.75" 16tpi thread now to check the fit before ordering anything.

Worked like a charm, perfect fit and I could wind it in all the way! I'm actually surprised at how well it worked.