Whilst I'm happy to accept a small degree of leakage from my British bikes the M20 gearbox oil loss can rapidly become damaging and it's best to try and control it...If you really want authenticity and run an M20 without a sealed main gearbox bearing I'd recommend checking the oil level frequently...Very frequently...About 250-300 miles is all you need for the level to drop from the correct level to a point where damage to the gearbox is possible....The sleeve gear bushes tend to be the first thing to go...
A rubber sealed main bearing (with the inward facing seal removed before fitment) more or less eliminates the leakage problem and reduces the need for such frequent checking and the likelihood of the need for a gearbox rebuild...One disadvantage of 80 year old motorcycles is that they pre date the introduction of modern oil seals!...The sealed bearing part number is 6205 2RS and being a metric bearing is readily available from any bearing supplier...
40 or 50 SAE engine oil is the correct lubricant for the BSA gearbox...Ian
According to the speedo, this box has done about 40 thousand miles in 22 years or so. I have a rebuilt one to replace it with so that'll go in & I hope it leaks less. The old one will get a sealed bearing.
Thanks for the correction Ian.
While we're here, another question. The sliding plate on my inner chaincase has one open & one closed slot to accommodate the securing bolts.
Parts lists show open slots on both ends so I can't get an idea from them. Which end of the plate, open or closed, should face which way? I can't see that it makes any difference but perhaps I've just had a boy's look.
Because I was trained in engineering sciences I look for anything that can go wrong.
SO when I do mine the closed end goes to the back lest a thrown chain possibly pick up the open end.
I also fit a loose nylon tube over the spacer tube to prevent a loose or collapsed chain eating through the steel
