Can anyone identify a thread form?

[Malcolm M]

Can anyone identify a thread form? Bolt diameter 7.8mm, 33 turns in 38mm (1.15mm pitch, 22 tpi) head 13.5mm across flats (13mm spanner won't fit, 9/16inch is very sloppy). Other parts of the equipment are held together with Allen bolts taking a 5/32inch key, which might be a clue.

 

[jmec87]

I would double check the pitch. Based on the diameter, it might be either 5/16 - 18 UNC or 5/16 - 24 UNF.

 

[Stick.]

Any chance it's a 5/16 British Standard Fine thread? That has a diameter of 7.87mm, 22 tpi thread pitch, and a wrench size of about 13.3 mm across flats (it looks like the wrench size for it may have changed over time).

https://en.wikipedia.org/wiki/British_Standard_Fine

 

[Tmoose]

inch threads = 60° threads
British standard (Whitworth) threads are 55°

Do the Allen bolts thread into hole the mystery bolt came from ?
And vise-reversa ?

 

[Malcolm M]

I think Stick has nailed it. My measurements are a little approximate- the thread is badly worn in places, the head has welding spatter and there is rust all over. Now I have to decide whether to track down a nut that fits, or file off the welds and replace with an M8 (which would also obviate the requirement for a 17/32 spanner).