I am trying to fit an air filter to a 276 carb on an M21. The thread is 1 1/4" x 24 TPI. I need to make an adapter on the lathe. The question is the thread BSF or CEI or ?????

Hitchcocks sell one for under a Tenner! Ron
https://accessories.hitchcocksmotorcycles.com/Amal-Carburettors/Air-Intake-Adapter/19215

email (option): ronpier@talk21.com

The problem is I am trying to use a period BSA air filter. I don't want a foam sock or other modern air filter. I have a BSA air filter off a 1956 M21 by which time they were using monobloc carbs. The bike I am fitting it to is a 1950 M21 with a 276 carb and BSA never made air filters for these other than the WM20 one on top of the tank. The filter has a female 1 3/4" x 24 thread. The 276 carb is 1 1/4" x 24 male thread. So the Hitchcock adapter won't fit the filter. I just don't know what thread type AMAL used? My guess is it's either BSF or CEI?

I have modified the internals of the BSA filter to use a modern filter element and it sill looks original from the outside. Where I live in the summer riding any bike without a filter is not a good idea. A number of roads around me are still dirt roads.

The Amal book only gives the information that you already know. It's a very fine thread for the diameter involved. It's not ME. I'm not an engineer, but since there's only 5 degree difference between the thread angles BSF and BSC, could you not cut one and try it and maybe alter it if it's the wrong one? Ron

email (option): ronpier@talk21.com

It looks to be BSF but can't be certain as even with thread gauges, I can't see a 5 degrees difference. Having old eyes is not good!

I emailed AMAL, so I will see if they reply.

Most likely to be CEI than any other thread form
For some weird reason CEI threads do not vibrate loose as easy as any other profile , so they say .

Hi, I had an adapter made for my cabby by a machinist,I am certain if take the bits to a machine shop they will be able to manufacture the part with correct threads as I think the threads are cut on a lathe.
Cheers Ian

email (option): Gawls62@icloud.com

The carb adapter is finished and am now running a air filter of a later BSA which had a monobloc carb.

For reference the carb inlet is 1 1//4" x 24 BSF.

The air filter is supposed to be 1 3/4" x 24 BSF but if you measure it, you get 1.80" x 24 BSF. How could they have made it .050" bigger?

The internals has been modified to use a modern filter you can wash and re-oil (K&N type).

After 71 years the old girl finally gets to breath clean air.

Well done, it would be nice to see some pictures so others might follow.

email (option): pes.sales@btconnect.com

OK, how do I post a picture?