The right gearbox ratios for a diesel motorbike is important. The bike has to pull away with ease in first gear and obtain a reasonable speed in top gear.

	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

Royal Enfield India 4speed Albion: Recomended stockest: www.hitchcocksmotorcycles.co.uk		Search for Enfield gearbox
Widely available and reasonable second hand prices make this an obvious choice when combined with a 4cm longer main shaft. Gear ratios can be changed as bellow. Be aware when ordering longer main shafts, that in 2002 this box went metric to accommodate new bearing sizes, bearing and shaft size changed from (imperial) 16.35mm to (metric) 17mm.

Royal Enfield Redditch 4speed Albion: Ł47		Click to enlarge
Brit boxes do leak oil, one of the reasons why they call them Royal Oilfield. How ever these days we have enclosed bearings and oil seals that keep the oil back. Just replace the old main bearing felt (inside case), and output shaft nut felt, with an oil seal. There is no reason for an Enfield box to leek. The Greeves 24RBS 5 speed internals will NOT fit this box.

Albion 4 speed trials gears.

The Enfield HG (1959 on) trials gears lowers the ratios for 1st, 2nd, and 3rd gears in an Enfield 4 speed box, leaving 4th gear standard. So if we up the sprocket to a 22 tooth it should bring 1st 2nd & 3rd back to normal, taking 4th gear in to overdrive. HG trials gears available from Hitchcock's mc's Ł85+ del and vat.

Pre 1959 Enfield H gear set also had trials gears. A bit different to the later HG gear set as it replaced the 18 tooth for a 17 tooth small 1st gear, and the 30 to 31 tooth larger 1st gear. Apparently the later HG gear set being chunkier, had a slightly larger out side diamater, the larger trials 1st gear would touch the alloy gearbox case, which is why they made the special cut small 1st and 4th gears.

Royal Enfield 5 speed Criterion: This new box and clutch sold for Ł477

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This box has to be the pre-unit gearbox of choice, 5th gear is 1 to 1 ratio and there are no alternative gear ratios, how ever 1st gear is a lot lower than the 4 speeds, so a bigger sprocket can be fitted to push it higher in to overdrive than a standard 4 speed (not trials). Like the trials gears, the longer main shaft is not available for this box.

Designed & developed in the UK by Dr. Stuart McGuigan.

A second hand 5 speed was sold on ebay for Ł250 recently, which is a good price.

Royal Enfield Clipper 250cc 4 speed Albion: Sold for Ł33 on ebay		Search for Enfield gearbox
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.

Greeves

Greeves 24RBS Silverstone 5 speed racing Albion 1964: ebay Ł247 28/1/07		Search for Greeves gearbox
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 Ł236.99 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 Ł2000 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

Triumph 650 pre unit / Trident 5 speed: Ł335 eBay		Search for Triumph gearbox
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 Ł235 just for the internals and around Ł100 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

Norton Commando 4 speed AMD: Ł120 eBay		Search for Norton gearbox
A good strong box made by AMD, Norton used these boxes for a few of there bikes as did AJS Matchless. Expect to pay anything between Ł100 to Ł180 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

Ariel B52: 10-Oct-05 from eBay Ł47.57		Click to enlarge
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 @ Ł30+ each, so check for wear before you buy. I'll be keeping hold of this box for a future project as the 6 spline shaft fits the Neb clutch perfectly, and possibly Jawa. Trials gears are available @ around Ł150.

BSA

BSA A10: eBay Ł14.00

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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.