I'm guessing BSA wouldn't have fitted something if they didn't think it was needed...Exactly why they thought they needed it is another question...
In BSA worksheets it's purpose is essentially described as an 'anti drain valve'...
However, neither the new cast iron pumps currently available as a replacement for the original or the Dutch 'ABSAF' high capacity oil pumps for the Gold Star have this feature...

Below is an extract from the 'Air publications' manual for the M20 which is taken from the BSA factory literature....

['there are two ball valves incorporated in the lubrication system to prevent the transfer of oil from the tank to the crank case...']...Ian

email (option): ian@wright52.plus.com

Thanks Ian. Interesting, seems that oil pump valve didn't come together with the new engine but a year later. 1939 parts book shows old bottom plate 65-2613. New plate 65-2625 is given in 1940 parts book.

Kalle

Does anyone have a part number for the little wire clip that stops the ball escaping? I'd like to source a few of these before committing to having a thousand made.

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

Drags have them Mark. Expensive little blighters! Ron
https://draganfly.co.uk/shop/72715/circlip-oil-pump/#65-2622

email (option): ronpier@talk21.com

With £2 post and packing plus vat that's £9.18 each!!!..Russell Motors did have the complete bottom plate...Ian:grimacing:

email (option): ian@wright52.plus.com

Thanks Ron, that's just greedy. I think I'll have a thousand made.

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

I have a 1937 m20 3000 orig. miles totally original 1000 serial # came with check valve at base of pump. I helped work on very early 1937 300+ serial # had 2 bolt sump plate had no check valve at base of pump hollow tube. I also looked at a late 1937 engine 2000+ serial # had a slightly different pump base check valve tube approx. 1/8"to 1/4'different location from mid year pump if I remember correctly but that was 20 yrs. ago when I restored my 1937.

email (option): millcrocfarm@gmail.com

Low and behold we now stock the oil pump ball circlip 65-2622

These we are able to sell at £2.00 ukp each

I just need to finish the rest of the pump now :(

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