Looks to be 3/8" imperial. Check the thread count -could be BSF -20TPI, BSW 16TPI, UNC also 16TPI. Appears to be too coarse a thread to be UNF @ 24TPI.
Thanks for the info. Its definitely not M10 - have tried a nut but it just drops on. Its from an Azenda compressor. I will measure the thread count shortly. Thanks
3/8 whit and 3/8 unc are both 16tpi, one will fit the other but they are slightly different threadforms, if its a critical part (which this doesn't seem to be) then it would be worth investigating it properly with thread gauges, but I come across this from time to time on old machinery and just use unc without problems,
With regards to the whit/UNC It also helps to know what machine it is and where it my have been designed and built.
If it was made in the US of A then it's " probably" unc, an old British machine would " probably" be whit. Doesn't always work but it's a good rule of thumb
Its the studs that attach a motor onto a compressor. Not sure if its American or British so ordered 20 whitworth and 20 UNC nylocs from ebay for £4 delivered this morning! Only needed 4 but worth having spares. @Nick67GT thank you very much for the kind offer and thanks to all for the advice. Andy
Nuts arrived today. Turns out they are BSW. UNC went on a thread or two before getting tight - sure they would have went if I wanted them to go mind you. Thanks again all.