Understanding historic threads

[Billington]

Hi, I hope you can help me, after purchasing my first classic British motorcycle I’m skilling up and have now come to the point where I need to better understand Witworth nut, bolt, stud pitch and threads.

My bike is a 1932 BSA and I need to order some longer studs to mount my carburettor which requires a Tufnol spacer. I’ve purchased a Witworth screw thread pitch measure gauge and measured the thread on the old studs, which appears to be 26G. The diameter of the stud is 11mm. My problem is that I’ve looked at the threading table on: http://historicmotorcycle.org.au/images/Thread_Tables.pdf
However it does not mention 26G, hence I don’t know how to specify the thread I require.

P.S thank to Chaterlea25 who has pointed me to the following website:

www.motalia.net/

[chaterlea25]

Hi Billington,
That sze is rather big for a carb stud
Usually they are 5/16in. x26 tpi when threaded into cast iron, or 5/16 x 18 into alloy

11mm is probably 7/16in. (11.1mm) x 26
Triumph for one maker used this size, BSA used 7/16 x20

So it looks like the cylinder has been tapped out ?
Is the stud stepped down to the carb flange size?
A photo would speak a 1000 words
If so you need a "friend with a lathe" to make custom studs or internal / external threaded sleeve to reduce to "standard" 5/16 studs

If you really want to to study threads ? go for it here 
https://www.gewinde-normen.de/en/cycle-thread.html

John

[L.A.B.]

First, it must be understood that not all British threads are "Whitworth".

Whitworth (or BSW) is only one type of British Standard thread used on British motorcycles.

I’ve purchased a Witworth screw thread pitch measure gauge and measured the thread on the old studs, which appears to be 26G.

26 threads per inch (TPI) is more likely to be CEI/BSC (Cycle Engineers Institute later British Cycle Thread) which is not a Whitworth thread and CEI/BSC is a common thread found on British motorcycles.

The diameter of the stud is 11mm.

I suggest you try to keep to Imperial measurements so 11mm is probably going to be 7/16".

Other threads commonly found on British motorcycles are BSF, BA and BSP, plus BSA also used some of their own special threads.

These websites should be of some help in understanding British threads:

http://www.sat.dundee.ac.uk/~psc/spanner_jaw.html#Background
https://britishfasteners.com/threads/index.html
   

[Rex]

First off, it's "Whitworth" and that's a coarse Imperial thread.
26G(??) I suppose means 26TPI or threads per inch. This isn't a Whit thread but usually BSCY as used on old pushbikes and motorbikes.
Lastly it's pointless measuring an Imperial thread in metric units. The most common threads in this sort of application are 5/16" and 3/8" BSCY, though in my experience carb studs are usually 5/16" BSCY.

[R]

My problem is that I’ve looked at the threading table on: http://historicmotorcycle.org.au/images/Thread_Tables.pdf
However it does not mention 26G, hence I don’t know how to specify the thread I require.

It should be pointed out that Chart No 7 on that link has omitted several of the common sizes encountered, including the one(s) you are most interested in.
To wit  7/16" (and 1/2") are simply not there  - and should be,  in 26 threads per inch, which is an unfortunate omission on a motorcycle based threads website !!

As already pointed out, this would make your studs likely to be 7/16" in cycle thread, which is a hefty size indeed for such things,
so it does sounds like they have been drilled out and tapped to a larger size, which will make replacement a little problematic.

P.S. Many studs in the motorcycle world can have different threads at either end, so the nut end may have a fine cycle thread,
and the business end into the head MAY have a coarser thread (BSF or Whit), your stud isn't like this is it ?

[iansoady]

Some excellent replies there. The only thing I would add would be that I have never come across a 7/16" carb stud on mainstream British bikes - all would be 5/16", BSC (Cycle) on the outer ends, BSW / BSF for alloy / cast iron for the inner ends as said. Normal carb flanges only have 5/16" clearance holes so 7/16" wouldn't go through them anyway.

I agree - please measure in inches not mm. If you have a cheap digital caliper this is perfectly adequate for the purpose.

It seems likely then that your studs have been made to cater for stripped threads in the head (or is it barrel? - side valve?) and are probably stepped. You may be able to helicoil the threads back to stabdard or have inderts fitted depending how much meat is left.

I think this is probably getting more involved than you anticipated. This is invariably the case - a five minute job turns into a major restoration. But stick at it!

[Billington]

Thanks everyone for your advice. I’ve decided to purchase a 7/16 CEI cycle threaded bar, that is 12 inch long with a thread of 26tpi in stainless steel, from ebay costing £30.

I hope to cut this to the required length. I’ll let you know how I get on.

My background is in computer science but I’m loving the challenges presented by keeping an old British bike on the road. I’m now using the tools bequeathed to me my grandfather. Thanks Gramps for buying good quality tools.

[R]

BSA swingarm bolts are 7/16", and would give an unthreaded area in the carb flanges
- which if threaded could fret away at your carb.



Thats a massive size for the task, the flanges must be huge to allow for that,
or have very little metal left if someone has oversized them.. ?

[john.k]

I recently salvaged an old brickies "jenny" wheel from a cleanup,and on pulling it down ,its been a NOS front Matchless wheel,not a chopped nut or thread anywhere........all the threads are 26tpi,and very closely fitted.......Unfortunately two large washers have been welded to the spindle ends ..........but bless those long ago brickies,the left the front brake in position ,so its still there,courtesy of the welds.