WM20's seem to go for around $ 5,000 US if running and up to $ 10,000 if they have genuine military provanance and have been fully rebuilt ( a bike is only original once and even that is debatable )
IT was part of the 1936 rationalization plan BSA got Val Page to oversee .
They are simple, low power ( 13.2 Hp @ 4,600rpm with peak torque
at 3,400 rpm ) & virtually indestructable.
Much of the parts will interchange with the B Series preunit singles so most parts are not a problem .
The mag/dynos are different to the B series but a B series one will go in & work just fine
If you are thinking about riding it then remember they come from the age when "gentlemen" rode bikes and you ride them at a gentlemans pace.
The gearbox is their achilies heel but you can fit a B series box in there .
The engine is slow, the gear change is even slower. They will do 45 mph all day & all night without any bother , 55 mph with protest and 65 mph under duress
The M range was the heavy weights designed to be hooked up to massive double adult side cars .
So if you can handle the roads at 45 to 60 mph , do not worry about oil drips and are willing to put up with its funny little fobles then you are in for a long & fun time
If you expect an oil tight bike that can do highway speeds with fine precision handleing aand actually stops when yo apply the brakes them you will be bitterly dissapointed .
I got mine around 1996 and have done several hundred thousand miles on it and once I stopped trying to keep up with the A 65's and ride at M20 speeds it has been a blast .
While the standard tool kit supplied is about all that you need for day to day riding you will find getting your hands on most of the special engine tools shown in MM's 54 Gold Star thread https://www.britbike.com/forums/ubbthreads.php/topics/889308/1954-bb-gold-star#Post889308
will make you life a lot easier.
And apart from them you must fabricate a single spring clutch tool or you will never get the clutch back together if it is running the original single spring clutch.
A lot of WM 20's run the 6 spring B series clutch but if you fit one you will need to space the primary drive sprocket out about 1/4 "
The riders handbook instructs you to check all Fasteners
EVERY time you ride the bike and they are not kidding as all of the nuts & bolts work loose and the one behind the cylinder on the timing side is "not easy" to get to .