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 Using the right gearbox for a diesel bike is a must! The bike has to pull away with ease in first gear and obtain a reasonable speed in top gear. If there's too little load on the engine at top revs she'll vibrate and waist fuel, so getting those ratios correct is important not only to your pocket but to your bikes credibility.

A ten horse power diesel engine will be fine with a four speed British gearbox as 3600 revs will get to 60mph which is all we can expect a small engine to do. On the other hand a big engine like a 1000cc will defiantly need a well thought out set of gear ratios, and the more gears the better.

By far the biggest problem with building a pre unit diesel bike is how to align all the sprockets. This is due to diesel stationary engines being wider than petrol bike engines. As Royal Enfield has the longest main shaft, this is an obvious choice.  

 

to offer information on different gearboxes, budget and high-end, looking at the cost of the box and the effective use in a diesel bike.  For now the page will look at pre-unit boxes, and BMW shaft types and CVT's will come later.

 

Royal Enfield Clipper 250cc 4 speed Albion: Sold for £33 on ebay   www.hitchcocksmotorcycles.co.uk  
Look upon this box as a pre unit 250 Crusader box, the GT 5 speed cluster may well fit it. Smaller does mean more fragile, and the 5 speed suffered from false neutrals.

Vincent

As this is an Albion box www.hitchcocksmotorcycles.co.uk would help with most parts.

It seems that there was a short run of 5 speed gear box internals supplied by Albion, these would fit various bikes that use Albion gear boxes. This was as an after market only product, and used by at least one Vincent Meteor (not Enfield Meteor) racing bikes. As the gears would be narrower than standard to fit them in side the box, we can assume that we can not fit a standard trials first gear, hence no overdrive. Finding one of these 5 speed gearboxes will be impossible, so I now give up looking.  

Amended: This 5 speed kit is now being developed by Hitchcock's motorcycles. Unfortunately It's not ready for sale yet.  I will update when I know more.

Greeves 24RBS Silverstone 5 speed racing Albion 1964: ebay £247             28/1/07
This box looks absolutely gorgeous when polished up, but the problem is it's just like the Enfield GT 250 5 speeds, the internals are a miniature version of the Enfield bullet box and the extended (Taurus) shaft will not fit. 

 

Greeves MX3 Challenger scrambler (retro fitted to racers) Albion : ebay £236.99      22/11/07  Also available in 5 speed
This is an Albion that shares no common parts with the Enfield box. The longer Enfield main shaft will not fit it, but the Enfield clutch may, as well as sprocket, (the Greeves sprocket uses a smaller pitch chain). Same bearings and seals as early Enfield. Expensive, rare and 5 speed more so.

 

For those amongst us that are fortunate enough to have a life where money is no problem there are 6 speed alternatives

This is a 6 speed replacement for the Greeves MX3 box used in Greeves racers. But nova racing assured me that a 6 speed for an Enfield amongst others is no problem.

So if you have £2000 plus carriage plus vat to put in to your box give Nova Racing a call. I'm sure they will make one with an extended shaft and make the ratios to suit your requirements.   http://www.novaracing.co.uk/albion.htm

Triumph 650 pre unit / Trident 5 speed: £335 eBay
I was looking at a Triumph pre unit gearbox and replacing the internals with 1970s 5 speed unit internals. But this is too expensive at around £235 just for the internals and around £100 for the box.

Sprocket alignment will be a big problem if your using a Triumph box with a twin cylinder engine as  the clutch is very close to the output sprocket. But it should be ok with a single or a vee twin engine like a Punson.

Norton Commando 4 speed AMD: £120 eBay
A good strong box made by AMC, Norton used these boxes for a few of there bikes as did AJS Matchless. Expect to pay anything between £100 to £180 for a good one. There is a 5 speed version available made by Quaff, although these are well rare and well expensive.   This being a 6 spline shaft should be good with a Neb clutch and possibly Jawa also. Different ratios are available as Norton changed them slightly for later 850 Commandos.

Ariel B52:  10-Oct-05 from eBay £47.57     http://www.draganfly.co.uk/

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Looks like a well designed box which uses a drum selector. The main bearing is an odd size,  hard to get and so a little expensive @ £30+ each, so check for wear before you buy. The 6 spline shaft fits the Neb clutch perfectly, and possibly Jawa. Trials gears are available @ around £150.

BSA A10:  eBay £14.00    http://www.draganfly.co.uk/ 

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BSA parts are way cheep compared to other Brit parts.

 I have been told that this is a good strong box.

Although this is a 6 spline shaft box it will not fit the Norton, Neb or Jawa clutches as the raised splines are wider.