Hi

I have a number of twist grip clamps where threads have pulled through

Can anyone confirm the thread type - it’s either 0BA or 1BA I am led to believe

Reason for asking is I want to helicoil them but the uni thread kits I looked at varied so much in price

0BA thread repair kit is £24
1BA thread repair kit is £48

You can see why I need to get it right

Hope someone can confirm

Br

Jo’b

email (option): jonnyob1@googlemail.com

The Doherty clamps fitted to Norton are threaded 3/16" x 28tpi according to the parts list - I assume it's a non-standard BSC (cycle)...Certainly 0BA ain't right on the Norton and I suspect not correct for the Amal clamps on BSA either.

Well I always thought the Amal grip screws were 1BA.(I just tested one with a 1BA tap). Certainly not 0BA which is nearly 1/4" and the size of the studs on our speedos (some guys claim they are metric but I don't agree on British made speedos, but it is the same size thread as 6mm)..... But I digress!

If any of the clamp threads are indeed 3/16" which is just over 20 thou smaller than 1BA, then a 1BA (28.2tpi) tap might clean them up without the need for hellicoils. Ron

email (option): ronpier@talk21.com

Were the Smiths speedos not originally made under license from Jaeger, a European firm? I understood that is why the mounting studs were metric together with the 1mm pitch thread that secures the bezel to the body.

Regards Jim

email (option): johnboywalton13@msn.com

Dennis Quinlan posted this drawing on the Smiths Chronometric Facebook Group.

It shows that by 1957 at least, the studs were 0BA but it can't show if they were metric in the early days...I suspect that pre-war (with the Jaeger drive) actually were and that the licensing agreement ended with WW2.

The 6mm stud size is pretty well identical to 0BA but I'm sure the nuts would have been imperial hex in UK. Ron

email (option): ronpier@talk21.com

I agree Ron. The dull chromed Norton nuts had imperial hex. without any doubt. The case studs are too crudely rolled to accurately measure.

There is a difference between the M6 and 0BA thread angle, but as you say Rik. To crude to worry about. Ron

email (option): ronpier@talk21.com

Thanks all

Will check when at Newley marsh where fittings & screws are plentiful

Br

Jo’b

email (option): jonnyob1@googlemail.com