I have a 54 M21. The gearbox action is imprecise with it easy to overselect and get the next false neutral. If I adjust the threads by the minimum amount it then is very difficult to get the throw long enough to engage first. In all cases I can't select neutral once stationary and running with the clutch in, though I can reach down and change by hand. The gearbox isn't noisy and doesn't much bother me out in the countryside where I live but I'd have to do something about it if I lived in London. What should I specifically look for to address this?

This can't really be covered in the space available...

There are numerous things that could be wrong with the gearbox from shimming of the shafts, wear of the numerous components and the setting of the box itself...

When the desired result can't be achieved by the small adjustments you have already tried a rebuild is usually the answer to return the box to the correct specifications throughout...

Note that the box is a 'slow' box...Any attempt to rush gearchanges will result in the selection of false neutrals...

Also, clutch operation affects the functioning of the gearbox so be absolutely sure that the clutch is in good order and set up correctly before getting into the gearbox itself...Ian

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

I had some similar problems, as have others from on this site. If you use the 'search' function you will find various threads discussing it. Basically it is down to the amount of wear that might be present and how much this interferes with the 'setup' or adjustment of the box. Take a long read at the article in the 'technical section' by Ian

http://www.wdbsa.nl/Gear%20Change%20Control%20Rod%20Adjustment.htm

Unfortunately it does require the box to be off in order to get access to the top inspection plate to see what is happening inside.

If you have got wear on any bushes or shims or to the slots in the dogs then these will all contribute to getting the correct adjustment very difficult. That, linked to the fact it is not a very good foot change box in the first place just about buggers things up completely.

Having grown up with Sturmey Archer Burman and Albion I have no problem with slow change. I think my Nuovo Falcone has a delightful gear change.
Clutch is set up well so I guess it's rebuild time, or more likely I use something else in Cities.....
Thanks both of you

The gearbox, if nothing else, displays the M20s 1930s heritage...It is basically the old hand change box with an enclosed foot change assembly added...

Triumph came up with their 'cam plate' operated gearbox by 1936 (the year the M20 was introduced) and it was the model from which most later gearboxes from other manufacturers were based...

This arrangement didn't appear on a BSA until the rigid A7 was introduced after the war and a look at the details of that box, the plunger A10 variant and the 4 speed C11G gearbox (which was the plunger box with different mounts) can only lead to the conclusion that BSA basically made an altered copy of the Triumph design...(which they developed further for the S. Arm models)..

The 'mangle' gearbox of the M20 is definitely it's 'Achilles heal' and is the box I personally least like working on...The overly complicated gear change arrangements, reliance on shimming and setting and the fact the box doesn't function correctly statically (it requires the inertia of movement for correct engagement) make it an awkward one to refurbish....Ian

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

So by 54 the gearbox is still essentially the WD item?

Yes, though the outer cover, inner cover and main case were altered, clutch actuation was altered, oil filling was done through the front cover rather than directly into the main case via a filler 'spout' and a speedo drive was added, being driven off the layshaft..

A lipped oil seal was also added outboard of the main case bearing..

Detail changes were made to the selector fork track and pins and the engagement dog tooth form and the mating teeth on the gears were changed to help engagement, but essentially the box retained the same basic design and operating principles and most internal components are interchangeable...Ian

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

Good Day,

Mine is 57 M21. The gear box has been fully overhauled with new bushes and sleeves, bearings, springs, new set of gear change forks and did the spacing, shimming and setting as the spec.

Perhaps some tips below would helps.

1) The gear change control rod, gear change pawl and the ratchet plate position must be set accordingly as the mechanism of the gear changing is concern.

2) Test and confirmed the engagement to each gear position by manually rotate and engage the gear via the rear wheel rotation. The adjustment of the gear position is by the Gear change control rod.



1) If one of the gear short, either 1st gear or 4th gear, please look at the "gear change pawl and the ratchet plate position ". Its either over or less 1 ratchet position.




1) Yes indeed, this package came with this gear box. How to deal with it.

In gear position at 1st - slow tap the gear down until there's small click feels. It's in between 1st and 2nd gear.

The small click due to the Gear control plunger - part number 15-4224 rest in the small hole at the operating plate.

If there's no click feels, the Gear control plunger need to be re-sharpen / the hole need to be re-drill / the spring need to be change.

2) If there's a stuck feel, release the clutch by quarter till the bike feels jerking and pull in back the clutch. Normally at second jerking it will be release and if it still stuck, with clutch in, move the bike front and back while tapping the gear lever and it will be release.
( All above with Condition : The clutch must be set and test at full stop spinning ( stop by hand ) when pull in )

email (option): roslansubri@gmail.com

Thanks, I think I checked the ratchet a couple of years back but I'll check again, once summer comes. The bike was featured in Classic Bike as a rebuild by Rick Partington. The engine is very smooth and I'm pretty sure he did all he could to get the bike mobile and reliable without spending more than he absolutely had to.
I've retimed it and put a normal lead acid battery and modern regulator in. It ate fibreglass batteries, in days....
Basically the gearbox is what us technical types term as "shagged".

I shoved a Triumph clutch in it and the action isn't nearly as bad. It doesn't make horrible noises and I can find neutral I could wander round the smoke on it now. I generally leave well alone unless stuff actually breaks. Does anyone know what the reduction ratio is for the speedo?

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