Which Countershaft Sprocket?

Last night was the first chance I've had to take my newly resurected
Mk III out on the Interstate - what a sweetie! Seems I'm running
about 4300/4400 RPM to achieve 70 mph. I think I'd like to bring that
rpm down a bit. I recall reading that all US import 850s were
equipped with a 20 tooth countershaft sprocket. I've run a 21 on
my '70 Fastback for over 20k miles & find it just right. Any opinions
on a 22 tooth (or bigger?) on a Mk III?

I'm currently running a 20 and an RGM belt they are giving me near 20 mph per 1000rpm. My Mk3 pulls it no bother.
Try the 22.
Cash

jumpjg said:

Last night was the first chance I've had to take my newly resurected
Mk III out on the Interstate - what a sweetie! Seems I'm running
about 4300/4400 RPM to achieve 70 mph. I think I'd like to bring that
rpm down a bit. I recall reading that all US import 850s were
equipped with a 20 tooth countershaft sprocket. I've run a 21 on
my '70 Fastback for over 20k miles & find it just right. Any opinions
on a 22 tooth (or bigger?) on a Mk III?

Click to expand...

I agree with the post by cash. Try the 22 tooth. I switched from a 20 to a 22 last year and like it, especially for highway cruising. Feels nice and relaxed. You lose some acceleration and 1st is a little tall but my 74 850 pulls it fine. 22 is probably too tall for a 750.

jumpjg said:

Any opinions
on a 22 tooth (or bigger?) on a Mk III?

Click to expand...

22T was the standard UK and general market countershaft sprocket size for the 850 MkIII, and is what I have fitted to my own MkIII. However increasing the countershaft sprocket size results in more stress on the gearbox.

I like 21t on the Mk111 - Overall gearing is the same as the 750 so if it is a less busy feel that you're looking for, then the 21t will have the same effect. Stroke and piston speeds are also unaltered.

There have been fears about gearbox loadings with the 22t sprocket and my personal feeling is that a standard Mk111 is a bit overgeared - Theoretical maximum is 124mph and they don't pull that !

An 850 Commando will of course pull from low down but maximum torque is at 5000 r.p.m. and I like the feeling that I am cruising just below that level because a handful of throttle there gives instant acceleration.

As you're probably aware, the main disadvantage of 21t is that you really need a half link in the chain to end up with 99.