Fork stanchion puzzle

[Steverat]

At the Stafford show today I was tempted by, and bought a very pretty pair of newly chromed stanchions which looked much like, and were sold as BSAs. On getting them home I discovered the diameter is different  (BSA = 1.25", chromed = 1.31") top nut thread is different (BSA = 20 tpi, chromed = 26 tpi) and the lower end thread is also different (BSA = 28 tpi, chromed = 26 tpi).

The chromed stanchions have diameter reduced to 1.25" at the lower end over a length of 7/8", just enough for a BSA bottom bush to slip seductively onto it - whereas there is no step in the BSA fork stanchions at all.

So there's no way for me to fit these on Josef's bike. The question is - what are they?

Any ideas?

Regards
Steve

[Slymo]

Possibly old Royal Enfield? Pictures would help.

[Steverat]

Thanks for the thought, I checked on images with google: RE as well as AMC Teledraulics and Norton Roadholders all have the lower bush secured by a circlip as far as I can see. But Triumph is threaded both ends. Could it be that? Sorry to lead a conversation away from BSA.

In a weird sort of way it might be historically interesting because it shows the difference between design rules in the two firms at the time (early sixties) - BSA using a variety of threads even on the same component whereas Triumph standardising on 26 tpi. I wonder why BSA used a finer thread at the lower end of the stanchion? Presumably concern about vibration?

[JulianS]

Sounds like you have parts from 1969/1970 A50 A65 or B25 unit single shuttle valve forks.  Threads will be unified.

First photo from 1969/70 workshop manual showing dimensions, second from 1969 parts book.

[Steverat]

Thanks Julian
Does your resource have any detail about the threads?
Steve

[JulianS]

The 1970 parts book shows both top and bottom as 1 1/16 x 28 tpi UNF.

[Steverat]

Thanks Julian.

The actual tubes are 26 tpi top and bottom, so I'm drawing the fairly conclusive deduction that they are something else. Maybe Triumph? Part number 97-1299 (fitted to the 1960-63 T120, TR6, and T100SS) has the same 26tpi top and bottom. But can I be sure....

[JulianS]

Maybe you cannot!

The Triumph parts are about 22 inches long I think.

[Steverat]

Thats right and the chromed tubes are  22 1/4", about 3/8" shorter than my genuine  BSAs.