[VicYouel]

The  three Sunbeams I worked on were 1932 onwards and had just about all BSF threads but whitworth for  the aluminium interfaces.

Conversely Triumphs and Nortons used cycle threads well into the fifties if not later and sensibly whitworth for studs.

[phutton]

Up to and including 3/8", the BSCy thread is 26tpi. over that diameter, 26 and 20tpi can be obtained. Sunbeams had changed from BSCy to BSF by 1937, so you are unlikely to find BSCy threads on your machine except in isolated instances.

P.

[Thomas]

Hi Paul, I got information that it could perhaps be British Cycle Thread (BSC) which was used on older British cycles and is a fine thread which matches nothing else, although it looks very close to BSF. I do not know BSC threads and have certainly no source in Germany. Various charts in the web talk about 26tpi whereas my studs show 20tpi. Indeed, 10mm metric matches very well, although not original, of course. But I have no better solution.
Cheers, Thomas

[phutton]

If it's 3/8" and 20 tpi, it seems it must be BSF, but it could also be M10 x 1.25 which gives a slightly larger diameter (0.39") and a slightly finer pitch (20.32 tpi). I think you have probably done the right thing!

P.

[Thomas]

Thank you, Steve! Yes, they are out of stainless steel and nicely manufactured. Maybe altered, indeed. I tried to find such a stud to lift the saddle by a good inch and keep the original nuts. The position for a 190cm fellow like me is not perfect. I now "cheat" with a 10mm metric stud. The result is not too bad.
Cheers, Thomas

[Greybeard]

Mine are definitely 3/8th BSF threads, Thomas, but they are stainless so could have been altered. The only other thread form with 20tpi at that diameter that I've come across is UNS - United National Special (or Supefine, depending on where you look) - an American thread that's not very common.
Unless Sunbeam did a Rover Cycles trick and cut their own unique thread form, which I doubt, it could just be a badly threaded example - perhaps the die nut wasn't adjusted all the way in?

PS Rover cycles used 25TPI which I understand is not an option on Imperial lathe gearboxes. A mate of mine, who is a stickler when it comes to that sort of thing (Rolls-Royce trained  ) actually modified his Colchester lathe to cut them and makes his own taps/dies. I used BSF on my 1909 Rover bike as over short lengths it's fine - or at least good enough for me 

Steve

[Thomas]

TIt is not BSW but "some kind" of BSF. The diameter is indeed 3/8. The difference to BSF is very small (I measured 20 threads per inch) but the nut runs only two turns over the thread. It's strange. 

[phutton]

I just checked the spares lists again. The 1936 list does not specify the thread diameter, but the 1937 list specifies a 3/8 stud and nuts. I would have expected the thread to be BSF, but it could be BSW.

[phutton]

OK, sorry about that! If you have one of the studs, can you measure the thread diameter, and also the thread pitch or number of threads per inch? We can then check against various books to determine what it is. Does anyone have a sample we could use as a pattern?

P.

[Thomas]

Hi Paul, the studs are definitely not 3/8 BSF. I have been at a special workshop for non-metric material. These threads do not match any threat gauge. 
Cheers, Thomas

[phutton]

I'm pretty sure they would be 3/8" BSF (British Standard Fine). Can anyone confirm?

P.

[Thomas]

Hi all! Can someone tell me what are the threads on the saddle studs? Cheers, Thomas