Mauro the usual suspect for a puddle of gearbox oil, is the bearing on the clutch end of the main shaft #57 which is an open bearing with steel shields. The answer with this is to fit a modern 'sealed' bearing and hook the seal out on the inside of the box. Another possible cause is wear in the sleeve gear bush #54. Of course all this attention will require a complete strip of the box. In the mean time, don't park it on the side stand and keep it topped up to the filler plug with ordinary engine oil....I use SAE 50. Ron

email (option): ronpier@talk21.com

Great advice, if I can mount a sealed bearing I shouldn't have any more problems ....

The exploded photo is also useful ... Can I ask you where you recovered it?


Thank you

email (option): bongiovanni1988@libero.it

It's from my 1944 air publication parts list. Ron

email (option): ronpier@talk21.com

And never ever use a thickened oil or grease in a BSA box .
The thick stuff can not get into the layshaft bushes which will flog out oval very quickly if you ride the bike any distance.
I did this to my bike on suggestions of some well respected owners only to find they rarely rode more than 20 miles in a day.
My 400+ miles in day killed the box .
The oil level is the slot in the bottom of the oil fill hole.

email (option): bsansw1@tpg.com.au

This slot of which trevor speaks. My gearboxes have a hole in the threaded section just below the top of the filler point. This would mean filling the box up near to the brim. Is that what you mean? I can't find any other markings.
I'm aware of the oil threads here. What do people put in for Ausralian conditions?

I use 75W 90 agtrans because I got a 1/2 a 44 of it at an auction when I was bidding on the drum pump.
Those who ride generally use Penrite gear oil or strait 40W oil when you can get it,
Those with trailer queens use semi fluid grease.
You can get away with it just so long as you don't ride the bike much and definately no longer than a hour .

email (option): bsansw1@tpg.com.au

I use straight 40 engine oil as recommended by BSA and always have...I also fit the sealed bearing (with the inner seal removed) as a matter of course during gearbox rebuilds...
That's about it and it has served well over a number of decades, multiple bikes and a lot of use...Ian

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

Thanks for these replies.
Trevor's right re opinions differing about suitable oils Down Under. Various authorities have recommended everything from semi fluid grease to 90w, never a mention of BSA's instructions to use engine oil. BSA probably knew something about it.
I use Penrite & oscillate (vacillate?) between 90 & 250. 250 in summer & 90 in winter.
I fill the box til it's about half an inch below the lip of the filler, should I put more in?

I have owned a WM20 for 38 years and always used SAE50 in the engine and gearbox, as shown in the army hand book, 50HD. Another army designation for this oil was M220, which again was SAE50. What some people do not realise is that SAE viscosities for gear and engine oils run on different scales and 50 engine oil is actually about the same viscosity as 90 gear oil, but without any of the additives that might harm any bronze bushes. Also for maintaining the bikes it made sense to use one type of oil.

regards, Richard