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Re: Carb bellmouth thread size?

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

email (option): ronpier@talk21.com

Re: Carb bellmouth thread size?

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.

Re: Carb bellmouth thread size?

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

Scan-20210910

email (option): ronpier@talk21.com

Re: Carb bellmouth thread size?

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.

Re: Carb bellmouth thread size?

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 .

Re: Carb bellmouth thread size?

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

Re: Carb bellmouth thread size?

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.

Re: Carb bellmouth thread size?

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

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

Re: Carb bellmouth thread size?

OK, how do I post a picture?

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