Good morning folks.
I have read that the standard B40 WD gearbox can be converted to that of a B25/B44 (or compatible model) giving an improved road experience. I have seen complete B25/TR25 gearboxes for sale online, but my cautious side overruled my tendency for an impetuous purchase.
I have come across related posts, but not a definitive "do's and don'ts". For example someone noted that gearboxes from OIF unit singles might be different.
I thought I'd ask the forum first about the best way to tackle it, what the pitfalls are, if any ....
Thanks, Philip
PS I have spent some time searching this forum for a definitive answer.
I had a B40 SS90 many years ago. The gearbox was the least of its problems ;D . What improvement are you looking for?
I use the bike on the road only. The bike pulls cleanly from rest in 2nd, but it feels like there's a ratio missing between 3rd and 4th.
I was thinking that I could swap the complete gearbox from a road B25/B44.
The final drive sprockets on the bike are 19 and 50.
Gearbox ratios from the BSA manuals, B40WD has 3.05, 2.36, 1.57, and 1
whereas the B25/B44 has 2.65, 1.65, 1.24, and 1.
My question - would it be a straight swap or does it require any machining, etc. ?
I'm unsure, but surely finding a complete B40 road cluster would be the simplest way forward , for all I know (without deep investigation into parts books) a C15 complete cluster may drop into the gearbox.
I can see now from those ratios what you mean. The WD want a lot of (well 3) low gears for all their convoy and cross country work and one high gear for road journeys so with only 4 to choose from you get that big gap 3 to 4. I drive an ex-Army Land Rover with a 4 speed box and its the same but on the Land Rover the fix is to fit an overdrive and use it to split by going 3+OD.
As Rhyatt says, a road B40 cluster is probably the best and safest bet. A gear box lock up on a bike is an instant off.
I'm away from home at the moment but RR covers this in 'The Engine'.
I don't think the early C15/B40 box is the answer hence the B25 comments. What I do know is that you can't mix the early and late components!
I bought an very nice D5 Bantam, it was cheap and great condition. The guy said "be careful with the gears" so I carefully tried a lap of his yard. The box locked up. I trailered it home, there were no signs of it having been apart (but it must have been) as 3rd gear was new and slightly wider - a later type. Looked identical in every other way. I suspect he had rebuilt the box and when it locked up he gave up but he'd fitted a D7 gear in the D5.
I very wary of mixing parts in gear boxes as the differences can be very small.