I'm trying to figure out what the thread is for a '69 B44 oil pressure relief valve. It seems to be 5/8" x 20 tpi. Is that possible? The only reference I can find for that thread type is a cycle thread. Is that possible or have I measured incorrectly?
I'd have to sit down and work out exactly what thread it is but is basically a pipe thread, either British BSP or US NS, and it is either tapered or straight. If the switch is used with an O-ring, it will be a straight thread otherwise the sealing is achieved because the switch thread is tapered. Are you looking to blank off the crankcase hole or are you after re-tapping the hole?
Apologies - I had a brain malfunction yesterday and got it into my head you were talking about an oil pressure switch and not the oil pressure release valve.
Up to 1969, the pressure release valve plug was part 67-1505 but, in 1969, it was changed to part 21-1972 - I assume the change was from a BS to a unified thread.
I've just found a plug and its thread appears to be 5/8" x 20 tpi which I assume is the same as yours. Unfortunately, UNF threads sometimes come in special forms (UNS) and I believe this plug may be one of them and is using a slightly finer UNF thread than the standard 5/8" x 18 UNF thread. However, you can get a suitable tap and die for this thread:
https://www.tracytools.com/5-8-x-20-tpi
I have both a 67-1505 and a 21-1972 plug. The 67-1505, which I took out of my C15 to try nearly fits. It's the 5/8" x20 tpi. The problem with it is it threads in part way but won't seat because the cavity in the crankcase hasn't got sufficient threads. I ordered the 21-1972 from a well-known supplier. What I got was a plug of the correct diameter but with very coarse threads that were closer to an acme thread in appearance. The engine is definitely a '69 as the serial number stars "BCxxx" and the crankcase halves have matching numbersß. What I'm seriously confused about is the parts manual clearly says I should be using a 21-1972 plug. I think I'm faced with having to get a plug made with fewer threads so it doesn't bottom out.
Ken
Eastern Ontario
Having looked at both parts now, I can see that the earlier part 67-1505 is 5/8 x 20 and the later part 21-1972 is 5/8 x 11 UNC; that is, a very coarse thread. Unfortunately, 1969 can be a difficult year sometimes because BSA made some big changes that year and its not always clear which version a 1969 bike was made to. For the original bike in question, the 1969 B44 seems to correspond to the slightly earlier version.
For 1969 only both B25 and B44 changed the prv thread to unf.
Pm me if you need one...