Britbike forum Forums Thread Like Summary - How to build A65 gearbox one-up three-down

Hi everyone,

I am a newbie on the forum, so apologies if the answer to my question is already to be found somewhere (although I have searched..).

I am rebuilding a basket-case A65, 1970 pre OIF. Just one question for the moment (probably many more later):

The gearbox as configured now is one-down, three-up. I'd like to build it one-up, three-down. Can anyone advise how I can do this?

Thanks, Nigel

A. Get resets and flip the shift pedal backwards.

B. Have a reverse-shifting camplate made for inside the gearbox.

There is no standard BSA components that will give you 1 up/3 down. It is either reversed lever, linkage, or a custom cam plate.

Maybe, but if you took a camplate to a machine shop and said you wanted one mirror image plate, the cost might not be so different.
I revised the ratios after some discussion with Allen, making the overall range wider. As John stated, races are only six or so laps long so whoever gets to the first turn first is likely the winner. Maybe different down under.
Here is a comparison of various gearbox ratios:

Quaife is perhaps the worst, with the bad dip in the 4th gear. The third shift (3-4) drops 754 RPM and the fourth shift (4-5) 1427 RPM
The Triumph five speed also has a slight dip in 4th. Third shift drops 1125 RPM and the fourth 1197 RPM. Triumph's 1st is 2.585, slightly lower than the stock A65 1st 2.513. Probably because the Triumph does not have as much low end torque.
Hill's has a good shape and is a close ratio box, starting with a lower numerical ratio than the stock four speed, 2.294 versus 2.513.
I made the ratios with first the largest that would fit in the box, 1st is 2.786. I thought about making the fourth gear 1:1 and fifth overdrive, but it made the RPM drop between 4th and 5th small and the 1st gear RPM drop large because 1st, 2nd and 3rd has to go through the 5th gear ratio to get to the high gear.

Yes, shift drum instead of plate. Printed one to check fit, now making gears in aluminum to check operation. Layshaft held by ball bearing on timing side so no shimming required.

My current setup uses an overal ratio of 4.3:1 (36:70 primary and 19:42 F.drive) that went really well with the close box, second gear was about 80mph, though whilst it was fine for me riding about it meant I was constantly slipping the clutch on larger ride out groups where your stop starting and crawling a lot. But the bike pulled it fine, and the bike didn’t accelerate any slower than with lower gearing, if anything it seemed quicker, I didn’t find a problem with the close 3rd… whether it’s needed, I can’t truer comment, it worked well with the cr box but the standard 3rd also works well with the cr box too.


I’d really like to try Dave’s gearbox, pennies are always the big factor. I get threatened with death quite a bit at the minute where funds are concerned.

It would be a lot easier to use a linkage

Using rose joints makes a linkage smooth, slop free and long lasting.
Another advantage of a linkage is the gear lever can be positioned in exactly the right position. with a direct lever one spline could be too high and the next one too low