hobot said:
Fairly common for Cdo racers to adapt a Triumph 5sp with its non Norton looks.
Probably not so common as it was as supplies of pre-unit gearbox cases dry up. You need to get good info on exactly what Triumph 5 speed donor parts you can use. And then you need to think what mainshaft and clutch you are going to use. This is probably the cheapest route, but you are going to take time to get info and get parts together, and you are pretty much on your own with setting it all up.
The lack of an overdrive ratio comment might be misleading. The fact is that the Commando gearbox has a 1 to 1 top gear ratio, and this is true in aftermarket designs for 4, 5 or 6 speeds. So you set your 'overdrive ratio' at the primary and final drive gearing (generally a big gearbox sprocket) and run in 3rd as non overdrive....it is not ideal and gives you a higher first, unless you adopt Hobot's granny.....
Quaife Gearboxes are supplied through Mick Hemmings in the UK...he builds them....and will give very good Commando advice for all applications. The Quaife website link is good for the pictures only.
TTi probably builds the strongest box, it is also the heaviest, I have a 5 Speed XHD TTi for race use on a 750 Short Stroke, this is probably the only Commando application that would get some real benefit from a 6 Speed HD, maybe, but I don't anticipate any issues with a 5 Speed.
As the others have said for road use, build a good 4 speed and stay near stock.
You don't mention your application or state of tune of engine, if it is standard the proposal to go 5 speed is expensive, and probably pointless.
Quaife or TTi may also be out of your proposed budget, you don't mention that either, a new box is not an inexpensive route and any second hand box that has had much use is a real gamble.