If your Beeza C15 is 59 plate , then it won't have any unified threads, they started to come in , on British bikes at least, in the latter half of the sixties. All the threads into ally will be whitworth with the other end of the stud being either BSF or Cycle threads depending on the job. Cycle threads are generally all 26 tpi so once you get over 1/4 5/16 sizes cycle threads will appear to be much finer threads.. The rocker caps for example will be cycle threads. most of the bolts in the gearbox clutch and crankcase area are if I remember rightly have BSF nuts. The spanner sizes are , as I'm sure you know , slightly different to the unified "AF" sizes. The Haynes manual for BSA unit singles is a particularly bad one if I remember, the frame illustrated in the manual is the latest type used from about 1968, not much use on a bike introduced in about 1958. If you have the gearbox down change the detent spring, they are fairly notorious for going and a pia to change.

i will also recomend the rupert ratio book it is easy to understand and will tell you what and when the different threads were used, on the early c 15 you will find that the case screws are 1/4 bsf and normally got stripped , the cause was loads of old( normally red hermatite) jointing compound in the holes which stopped the screws from tightening the case properly , the result was the good old british oil leak, then the mate down the said he could tighten them this result was even more oil as the threads were now stripped, i always run a tap down the threads you will be suprised how much crud and bits come out ,, will

I think I would ask myself just why they would use BSC on a part like that as there is no gain. On cars I have seen finer pitches used on larger than 1/4 diameter high tensile bolts – big ends and mustn't be reused. I couldn't get replacements, no longer available so had to refit. The engine blew up rather suddenly on the M6. Visibility none existent as oil all over the screen and locked rear wheels at a speed I shouldn't have been travelling at. It put a good 1/2" or more kink in a massive crankshaft.

The nuts were also longer than usual.

Might help identify when and why they might be used.

John

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Lucky you are not working on an early DOT. I have just done some work on parts for one. 7/16" 25 TPI anyone ?

Tracked it down to some sort of Admiltary thread.

My 1929 BSA has a mix of 26 and 20 TPI cycle thread and I use UNF for the 20 TPI if the correct cycle stuff is not available, the other parts are a mix of WHIT and BSF.

All BSW and BSF threads have a thread angle of 55deg. BSC threads should have a thread angle of 60deg. BA threads have a thread angle of 47.5deg. BSB threads have a thread angle of 55deg.

Most of the threads on a pre 1968/69 BSA/Triumph machines will be BSW or BSF with some BSC and some BA sizes which were mostly on the electrics and handlebar fittings. After that time you can get UNF/UNC, BSW/BSF, BSC and BA as original fitment on the same machine. No wonder the industry went to the dogs!

As you've probably found out the Whitworth spanner sizes stamped on the spanner no longer relate to the thread sizes of the bolt/nut. This is because during WWII, to reduce the amount of material used to make nuts and bolts, the hexagon sizes were reduced by one size. In other words a 5/16 Whit. spanner size is used to tighten a 3/8 bolt/nut. The original hexagon sizes were calculated so that the length of the spanner used would supply the correct amount of torque for the thread size. UNF/UNC spanner sizes have no relation to Whitworth spanner sizes apart from chance matching of some sizes.

I think this link will give you all the info you are likely to need on thread profiles **LINK**

Edited By JonBerk on 21/11/2015 22:45:30