I have snapped the bolts that hold the exhaust manifold to the inlet manifold... I have managed to remove the snapped bolts.
Today I measured the thread and it equated it to 5/16 with an 18 TPI
Which according to my chart is 5/16 Whitworth.
Normally I get things wrong, but today could be my lucky day, am I correct?

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You can’t teach experience

There are no whitworth fasteners on American cars. SAE Fine and SAE Coarse, yes.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

I have just run a 5/16 BSW tap through the manifold and it feels good to me. Playing more with the electrical side of life, rather than the mechanical, I have spoken to the machine worker in the shop and he explained there is a 5 degree difference thread profile between BSW and SAE.
I guess if you would have all stayed within the Empire I wouldn’t be having this problem
Anyway, I had already ordered two 5/16 BSW bolts, so I will see how I go.

Thanks for your advice.
Ryan

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You can’t teach experience

And a small note: *please* make a clear note so you can show this to next owner if you ever sell that car and as future reference so you don't get frustrated trying to put on wrong nuts on oddball stud.

Next time use correct thread that *was* used on this stuff to keep confusion down.

Cheers, Wizard

Well on further investigation it turns out that this is indeed a 5/16 BSW thread.
I can only think that some where along the line someone tapped it out to this standard (only if they would have wrote a note and passed it on)
Thanks for your advice 65Dodge100 & Wizard

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You can’t teach experience