Hi,

I am assembling my Dutch Army BSA M21 project but it's not clear to me how many teeth the engine sprocket should have. I know it's not the same as an M20. My M21 will not have a side car, I will drive it as a solo mc. Can anyone tell me how many teeth the engine sprocket should have?

Regards,
Bastiaan

email (option): wdmotorcycles@gmail.com

Twintig tanden

email (option): John.kater@chello.nl

Yes when I built my engine into 600cc I tried a 21 tooth sprocket, but found it a bit too high geared for my liking. I changed it to 20 tooth and prefer the the engine being a bit more buzzy and more suited to convoy duties. Ron

email (option): ronpier@talk21.com

Ron, I did not know you understand Dutch. 😂

John 👍

email (option): John.kater@chello.nl

Thank goodness for Google Translate John:wink: But I have already learnt some very important words in Dutch from Leon and Henk for 'Velcro' - 'Lathe' and 'Current Bun' :thinking_face: Ron

email (option): ronpier@talk21.com

Klittenband draaibank en krentenbol :+1:

email (option): ahum@quicknet.nl

:grinning: Thank you for the reply's! I will look for a 20 teeth sprocket. Now I only need a M21 piston.

Regards,
Bastiaan

email (option): wdmotorcycles@gmail.com

https://www.ebay.co.uk/itm/BSA-4-Lobe-20T-B-M-Group-Engine-Sprocket-OEM-15-1564/372373185249?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m1438.l2649

Ron

email (option): ronpier@talk21.com

This item is out of stock. ☹️ Try BSA de Groot. 👍😅

email (option): John.kater@chello.nl

Your final selection will also depend on whether you have an 18T or 19T gearbox sprocket fitted...I think most of the comments so far relate to British WD machines which were fitted with an 18T gearbox sprocket.....Ian

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

Hi Ian,

Thank you for the reply.

I have got a 19 teeth sprocket on my gearbox. What would you recommend?

Regards,
Bastiaan

email (option): recce_van_niel@hotmail.com

It rather depends on what you use the bike for most of the time...
If your ride a lot in traffic, in town, or live in a hilly area 19T...If you live in a less hilly area and do more main road riding in more open country I'd go for the taller gearing of 20T...Ian

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

Also depends on how fat you are. I'm about 12st all up with clothing. 20 teeth for me in the hilly Welsh Marches. I've tried both.

email (option): jeremy@clogmaker.co.uk

Hi bastiaan

I run my relatively low compression m20 on a 20tooth

It accelerates even slower than before but delivers in my opinion a cruising speed with less buzzing from the motor & vibes

It does however feel it on hills - not so much a problem where you live

The m21 motor I have in mind for my 1940 bike I will start with a 21 tooth - i think it will take it

Standard gearbox gearing retained

Br

Jo’b

email (option): jonnyob1@googlemail.com

Hi Jo'b (and others),

Thanx again for the advice.

I have found out that it was very specifically mentioned in the contract for the Dutch Army WM21's which sprockets should be fitted to the engine and gearbox (both 21 teeth). I will add a photo I have made from that contract and from the BSA partslist for comparison. The contract is in Dutch, but the gearbox ratio's are easily compared with the 1940 partslist.

Regards,
Bastiaan

https://imgbbb.com/image/p9KsJ


https://imgbbb.com/image/p9wwU

email (option): wdmotorcycles@gmail.com

I am meant to be getting a BSA M21.
However, the gearing is freaking me out.. especially as if might be geared for a sidecar.
Is it the crankshaft sprocket that needs to be changed, of the gearbox sprocket please?

It's the engine sprocket that is far easier to change. The gearbox will probably have an 18T. and the consensus is either a 20 or 21T engine sprocket and for the extra teeth you will probably need to extend the chain by 1/2 or 1 link. Ron

email (option): ronpier@talk21.com