Sprocket size???

Sprocket size is very much up to the rider. Sprockets between 19 tooth and 24 tooth are listed in the parts list. Back in the day we would fit big sprockets, like 23 teeth, to improve fuel economy. 21 should work well. 99 links is correct chain length for a 21 tooth sprocket.

There was a nice article by John Cracknell in last months Roadholder. He swears that his machine has been transformed by using a 23 tooth final drive sprocket. Depends if you prefer comfortable cruising or, as in your case, faster acceleration with higher revs.

Ran a Mk 3 850 in Kenya for twenty years which had been a police bike used mainly for escort dutyfor the first President , Kenyatta , and during rebuild after buying it from the police found a 20 tooth g/box sprocket fitted. Rode it for a while with this 20 tooth sprocket but found engine revving high on long journeys so changed to a 22 tooth which made a big difference. Recall engine revs dropping 500 rpm at cruise speed of 70 - 80 mph. Drop off in acceleration was hardly noticeable.

Over the years I changed my 750 from a 20 to 21, then I put a 23 toothed sprocket on years ago, and it was the best thing I did. It transformed the bike, with better performance, more comfortablecruisingspeed and revs, and better fuel consumption.

I recently sold an 850 MkIII to a friend who wanted it up-geared, and I put a 24 toothedsprocketon for him. I thought this would be over geared but the bike is excellent for the long journeys he does on it. I rode it locally and the bike pulled the gear easily.

I need to ride his bike a bike more to see what the 24 is like, but I would definitely recommend a 23

Hiya, just to add twopennorth to this debate, don't forget that if you go for taller gearing ie. a 22 or 23 tooth gearbox sprocket you will bring the engine revs down for a given speed as several writers have pointed out, but the downside to this is, that this may put the engine into its heavy vibrating period at a crucial speed like 30mph in 3rd gear and 50mph in top which may make the bike a pig to ride at "favourite" speeds.

Cheers Alan

Increase the size of your gearbox sprocket at your peril........

John Hudson explained to me over tea and fruit cake a long time ago that the Commando primary drive ratios are wrong - the clutch is too large compared to the engine sprocket (take a look at the Dommie ratio). This was partially dictated by specifying an off-the-shelf ex fork lift truck diaphragm clutch at the design stage to replace the overstressed AMC clutch.

Fitting a larger than a 19T gearbox sprocket to compensate for the poor factory choice of primary ratio loads up the already highly stressed gearbox and you will suffer from very expensive additional wear of the pinions - the larger the gearbox sprocket and the worse it gets.

A major advantage of fitting a belt primary is to change the primary drive ratios (to say 2:1) allowing you to run a 19 or 20 tooth gearbox sprocket at the relaxed engine speeds you seek. This speeds up the gearbox and reduces the loadings on the pinion teeth - every little bit helps in keeping the box intact. The AMC gearbox is a lovely box for a 500 cc engine but not really up to the job of coping with Commando torque loads.

Another method would be to increase the size of your engine sprocket but that will require a longer triplex chain - and is there sufficient room for it all?

The Commando is a great but deeply flawed motorcycle.

Jim

With an 850 and all that extra mid range torque that can pull a higher gear I'd up the gearbox sprocket to 22 at least. Assuming the box has the layshaft ball bearing replaced with a higher spec or with a roller then it will survive fine unless its subject to abuseevery time you take off. I have a 750 with a 35/72 belt primary and 21T sprocket. That's 4.11:1 or close to a 22.5T. With a 110-90 x 18 rear it runs about 18.1/1000. It definitely can't be called 'long legged' with that gearing but 4000 rpm at just over 70 is fine. At 4500 and just over 80 you know its working. It also has a PW3 cam and an overworked head but its acceptable at lower speeds even without the extra 850 'oomph'. What it won't do is climb a 1in 6 hill in top at 40 mph or pull strongly from 30 mph in the same gear.I do use M-Ways and Duals quite a bit so it pays off for me.

I doubt you will have any issues and certainly if you spin the engine up thru' the gears the box should/mayhave an easier life? Using the engine like a 'Twist and go' from low speed in high gears might be worse on the box.