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...
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.
