Fobco is probnably a British supplier, so think in terms of Imperial sizes, not Metric.
First thing is to measure the OD of the stud, probably 5/16″ at a wild uneducated guess.
If you are a woodworker, you are unlikely to have thread gauges, so must do a bit of “suck it and see”
Try to count the threads over, ideally, an inch, but may have to be less.
Another WILD guess, that the threads will be
1/4″ BSF will be 26 t p i
5/16″ BSF will be 22 tpi
3/8″ BSF will be 20 tpi.
If the threads look to be coarser than these, they may be BSW, in which case
1/4″ BSW will be 20 t p i
5/16″ BSW will be 18 tpi
3/8″ BSW will be 16 tpi.
Knowing what the diameter is, and the pitch of the thread, if you can get a nut of that size and thread, try running down the thread. Whit or BSF may be hard to find, and expensive.
You might find UNF or UNC at places like Halfords, but probably at a price for just a couple!
It is possible that threads might be Unified, in which case UNF nuts might be easier to come by to try as a gauge.
1/4 UNF will be 28 t p i
5/16″ UNF will be 24 tpi
3/8″ UNF will be 24 tpi.
If UNC
1/4 UNC will be 20 t p i
5/16″ UNC will be 18 tpi
3/8″ UNF will be 16 tpi.
Unified and Whitworth form threads are not interchangeable, the thread form is different, as are some of the pitches..
Grease nipples are likely to be BSP, 1/8″ BSP will be 0.383″ OD x 40 tpi; 1/4″ BSP will be 0.518″ OD x 19 tpi.
Guessing again, the cover retaining nuts may well be knurled rather than hexagon, in which case are MOST unlikely to be available commercially, except POSSIBLY as spare parts. Either way, spares or specially made, costly, no doubt.
HTH
Howard
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