Made some progress (thanks to Rick for the data!).
In addition to the early M20 (standard, trials) gear set there are 2 other internal ratio selections: medium close (scrambles) and extra close (road race).
Usually, these parts can be swapped directly over to convert an existing box to the other factory ratios, or to create a selection that was never officially offered but may have some advantage.
Obviously, any gear can't just be put anyplace: the known-to-work swap uses only mainshaft gears on the mainshaft, etc. In addition, 1 mainshaft gear and 1 layshaft gear contain 2 individual gears and must be exchanged for equivalent.
This makes a possible total (without surgery) of the 3 factory sets, 1 extra wide ratio set and 2 extra close ratio sets for a total of 6.
The .xls accepts input for engine sprocket, tire diameter, transmission sprocket, engine RPM, clutch sprocket, and rear sprocket tooth counts. It returns RPM drop and % on each shift, MPH in each gear, overall gear ratio.
E-mail me at sales@victorylibrary.com for a copy.

email (option): sales@victorylibrary.com

Useful information to know and worth putting together..but the problem is the alternative gear sets are virtually non existent for these early boxes..in 40 years I've had one non standard set...Ian

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

I have a few early B group alternative gears but only one side

email (option): ahum@quicknet.nl

An old thread, but one I'm interested in. I acquired a 66-3219, which is part of the early gearbox close or extra close ratio setup. I'd been looking and this came up so I bought it. This is for my salt racer, as I raised that despite the long run up that land speed racing has you still,need enough power to bridge the gap between third and fourth. This year, with my engine not setup correctly, a result,of my normal last minute working, I was hitting 72mph in third, but the jump to fourth also resulted in that speed... Now there were lots of other problems, valve spring seat pressure for one!mwah too Low, now have some beehives give about 130lbs seat pressure, but still I've realised in need a slightly closer ratio setup. So I'm on the hunt for 66-3220, the 19T/20T layshaft gear which mates with 66-3219 which I have to lift all intermediate gears a little. I've realised this hunt is likely to be potentially infinite, but you never know. I'm considering if I coiled have a special made, ie machine off the standard 17T from 17T/20T gear and have a 19T made to mate to that, or simply get the whole thing made. Both options sound expensive bit perhaps the 19T hybrid would be within reach? Would anyone else have an interest in this if I looked into it with some local (to me in Melbourne) manufacturers?

email (option): dickie.bobbie@hotmail.co.uk

http://www.performance-engineering-services.co.uk/mark/Gear%20calculations.xlsx

This is something I did years ago whilst developing a five speed A65 gearbox. Just change the values in the boxes to suit what your using it for.
I found MPH in each gear for a set RPM made it easier to use.

email (option): pes.sales@btconnect.com

I found a BSA wd type gearbox at the original George Yeomans Emporium at Drayton Mill in around 1977 marked MCR and upon inspection it did indeed have the Medium Close Ratio gear cluster. I bought the gearbox and the MCR ratios are now in my M.21.

email (option): keithchandler@clistandchandler.co.uk

Keith,im wondering if you found an M22 or M23 box?

email (option): fuzzyempire@hotmail.com