falcor75
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| posted on 17/2/22 at 06:44 AM |
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Non metric threads? help!
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. 
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HowardB
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| posted on 17/2/22 at 07:29 AM |
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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
Howard
Fisher Fury was 2000 Zetec - now a 1600 (it Lives again and goes zoom)
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JimSpencer
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| posted on 17/2/22 at 08:31 AM |
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Hi
I think that's Metric just to be awkward.. M70 x 1.5?
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Mike Wood
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| posted on 17/2/22 at 08:43 AM |
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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]
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padrc66
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| posted on 17/2/22 at 11:55 AM |
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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
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Dingz
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| posted on 17/2/22 at 03:01 PM |
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Sounds about right, pipe threads are related to the bore of the pipe not the od
Phoned the local ramblers club today, but the bloke who answered just
went on and on.
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falcor75
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| posted on 18/2/22 at 07:46 AM |
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Thank you everyone, I am 3D printing a test piece with the 2.75" 16tpi thread now to check the fit before ordering anything.
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falcor75
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| posted on 18/2/22 at 10:55 AM |
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Worked like a charm, perfect fit and I could wind it in all the way! I'm actually surprised at how well it worked. 
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