Series A Torque Arms - Geeks & Anoraks only

[A_HRD]

I'm making some Series A Torque Arms and I want them to be right, i.e as per original.

I already have the correct length between centres of the 2 holes at the extremities (thanks Bob S) and the position of the spring anchor threaded hole - but its the thread itself that I am uncertain about. I know that the postwar ones have a 2BA thread, but I heard somewhere (can't remember from whom) that the Series A ones had a 3/16 BSW thread and had a square locknut on the inside of the torque-arm to lock-up the spring-anchor screw.

Can anyone with a known original torque-arm confirm this please? And while I'm at it, what form did the original screw-head take please?

Peter B
Bristol, UK.

 

[Albervin]

My 1938 model has 3/16 Hex bolts. The nuts are hex not square so maybe changed sometime when it was rebuilt in 1957. They seem to have been "made for purpose" as the bolt screws in just far enough for the nut to be flush with the end of the thread. This leaves just enough room on the outer end for the spring to have clearance between the torque arm and head of bolt. Hope that is of some help Peter.

 

[clevtrev]

I`ve got a Greek anorak, but I don`t think it`s worth much, It`s got one short arm and one long arm, so it might not be of any use for dimensions. The threads are still good though.
I could also add, `it seems the wheels are now coming off`.

 

[mercurycrest]

Peter,
My January 1938 Comet has what looks like, without actually removing and measuring, 3/16" Slotted, Fillister headed screws with hex nuts.
Cheers, John

 

[A_HRD]

Thanks go to Alyn and John for very helpful posts, as ever. Thanks too to Clev Trev for the laugh - as usual. I already have a few anoraks, but all the threads have gone so I'll consider your offer. Then again, anything Greek is pretty much worthless...

It would be nice if at least one more 'A' owner would come forward with his take on the torque-arm thread before I drill and tap mine. 2BA and 3/16 are to all practical purposes the same diameter. Its just that 2BA is about 31.4 tpi and 3/16 BSW is 24 tpi (much coarser) - so which is it?

Peter B
Bristol, UK.

 

[John Appleton]

Peter, we recently rebuilt a series A comet dating from 1936. I can confirm that the threads in the torque arms were 3/16" BSW, and the screws fitted when we got the bike were made from brass, and have a very short thread which only just protrudes through the arm, and a long plain shank for the spring to locate on. The screw head is a large flattened dome, slotted to take a screwdriver. There were no locknuts fitted, and not enough thread protruding to take one.

John

 

[A_HRD]

A Torque Arms

Thanks John for the confirmation.
3/16 BSW it is then. And probably a head shape similar to the post-war bikes...

Peter B
Bristol, UK.

 

[Black Flash]

I still have an old anorak here in dark green, if that is any help for you Pete.

 

[Robert Watson]

As my A has seen several teenage boys in charge, I cannot be definite, BUT, in the pile were a pair of screws much as John Appleton described, I can photograph them and send to you if you like, I may start it up on Sunday ???

 

[Howard]

Threads (scuse the pun) like this, start me wondering. Are we applying our understanding of modern day mass production techniques to bikes that were all "one offs" - I don't know. Just being Devil's Advocate.

I have a vision of "Old Sid" being responsible for tapping the holes, and making the special screws. After the first five bikes, Sid breaks his 2 BA tap, but he's still got bikes to produce, so he uses his 3/16" tap while the apprentice goes out for a new 2BA tap. Sid then makes 3/16" screws to suit the tapping. Are these non standard, new standard or factory produced specials? The rider with 3/16 screws will probably never know there was a 2BA screw, and vice versa.

H