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Wolfie01/11/2015 23:11:46
502 forum posts

I'm assuming that on a Beezer I'll find BSC screw threads as opposed to BSW/BSF. My question is WHERE I'm likely to find them? I mean are they used across the whole bike or just in certain areas?

Bob Rodgerson01/11/2015 23:54:28
592 forum posts
174 photos

BSC Trheads are likely to be on the cycle parts i.e. the frame. BSW is unlikely to be used at all unless your BSA is a Vintage bike. They tended to use whitworth threads for things like cylinder hold down studs in the crank cases, the coarse threads being able to take more tension than a finer thread such as BSF. BSF if used would more than likely be in the engine, However you could find that in bikes from the late 60,s onwards that a lot of the threads are UNF as a lot of bikes were exported to the USA so they used UNF threads.

Sorry if I have confused you, I suggest you buy a thread gauge and carefully check each bolt as you remove it.

Believe it or not my old Humber motorcycle of 1927 vintage has BSC threads, BSF and Metric.

stevetee02/11/2015 01:50:31
135 forum posts
14 photos

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.

James Alford02/11/2015 07:19:07
378 forum posts
73 photos

If you have not already bought a copy, or at least heard of it, I would suggest having a look for a copy of Rupert Ratio's C15 book. I found it invaluable when I rebuilt my machine. It is full of practical advice, detailed guidance and drawings, and clear, reliable, step by step guides to dismantling and rebuilding the engines successfully. I also have the Haynes manual and, by te comparison with the Raupert Ratio book, it is very poor.



will hawkes02/11/2015 08:56:43
19 forum posts

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

Wolfie02/11/2015 23:01:46
502 forum posts

Cheers, already got the Ratio books on order

Wolfie21/11/2015 16:01:37
502 forum posts

OK I have made a start and the first thread I've come across is a stud under the saddle. It just holds a bracket on but I'd like to run a die down the stud as its rusty.

The thread is 1/4" across (well actually 240 thou) and its 26tpi according to my thread gauge. How can I tell whether its 1/4" BSF or 1/4" BSC.

I know ones 55 deg and ones 60 deg but how do I tell. They are slightly recessed so I cant look side on with the thread gauge

JA21/11/2015 16:09:20
962 forum posts
52 photos

Really you cannot. A shadow graph instrument could help but....... If it was not on a BSA I would say it was Cycle since it is on the frame. However my little experence of BSAs is that they were a law unto themselves.

To make matters worse a BSF nut on a Cycle thread will work loose.

This is where a BSA spares book/list is invaluable. I think The National Motorcycle Museum will sell you a photocopy of one.


Edited By JA on 21/11/2015 16:14:38

Ajohnw21/11/2015 16:26:01
3631 forum posts
160 photos

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.



Nigel McBurney 121/11/2015 17:19:36
727 forum posts
3 photos

Cleaning rusty threads,avoid using a die,only use it as a last resort, dies do tend to remove some metal,make up a powerful wire brush unit, I use a 2hp motor,1440 rpm with a 8 inch diameter 1inch wide wire brush,this really removes rust but does not remove metal,the brush does not slow down no matter how hard you press the work onto the brush,need goggle though, although I have angle grinders with all types of wheel,I prefer the wheel on a motor, and I have had it 35 years,worn out a number of brushes. I can never understand why a cycle thread was ever introduced,26 tpi brass thread was already in use. with worn threads on alloy castings the answer is to helical them.I can usually sort out most threads,but the difficult ones are cycle /1/4 bsf and no longer having access to a shadow graph makes matters worse, I personally like Greeves,all their bolts were bsf but there were problems with bought in items,e.g. handle bar lever clamp bolts were 1 BA ,at least the Villiers engine was whit/bsf except for the threaded ignition coil pick up was bsp. I only rode in trials,and can remember the early trials C15s and tiger cubs,they were good comp bikes, except that you were lucky if you finished an event ,reliability was terrible ,electrics and pistons were the main problems,though I do remember one chap from a london club who used to ride mainly in events held on war department ground,he had a bog standard trials C15, a real rattly,leaked oil like a sieve,the bike was covered in oil from end to end never cleaned but just kept running long after most riders gave up on C15s.

Wolfie21/11/2015 18:35:36
502 forum posts

I have a spares list for the bike, it obviously lists the parts and part numbers, what it doesn't do is give any dimensions teeth

Neil Wyatt21/11/2015 19:15:03
18140 forum posts
713 photos
77 articles

Make a cup around it using blu tack, and fill it with vinegar or similar.


Chris Evans 621/11/2015 19:33:16
1725 forum posts

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.

Dave Attwood21/11/2015 21:20:24
5 forum posts

I have a CD with all C15 service manuals, parts etc etc .. yours for the postage

PM me


JonBerk21/11/2015 22:43:49
22 forum posts
2 photos

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

Chris Evans 622/11/2015 10:29:00
1725 forum posts

Add in BSA use of one size down hex sizes to access things like magneto nuts to put more doubt in your mind. I have used 1/4" BSF on cycle parts for years with no problems, with the tolerance of thread forming it is near impossible to tell the difference.

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