Im trying to identify what gearbox is fitted to my WM20 and a parts or assembly diagram. Can anyone help. I cant see any numbers on it at all.

Post war M-B? Ron
https://www.draganfly.co.uk/bsa/bsa/a710--b3133--c101112--m202133/category/902-b-m-group-1938-62-plunger-rigid

email (option): ronpier@talk21.com

Cheers that looks like the one.

It's essentially an altered version of the WD box, with the speedo drive incorporated into it, the clutch operation tidied up (with resultant changes to the outer cover) and a proper oil seal added at the sprocket end. Oil filling was done through the circular cover on the front of the box and as a result the oil filler was removed from the main gearbox case...

Internally it's virtually the same as the WD box and operates in the same way..The main difference is the oil seal, the worm gear machined onto the layshaft to drive the speedo and the associated speedo drive parts...

None of the castings are the same as the WD box but all of the internals are interchangeable, including the worm drive layshaft...Ian

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

Thanks for info Ian. I’m trying to get down to why it shifts I to o ok the the higher gears easily enough but I struggle sometimes to change back down. It’s crunching . 1st gear at a standstill is also difficult to select. Triumph 4 spring clutch conversion is new and not dragging.

My iPhone just had a spelling fit sorry