sprocket confusion

I want taller gearing on my 71 roadster, but a guy at a shop told me that I can't go bigger than the 19T countershaft sprocket because it would rub (on what, I don't know). My parts book shows sprockets up to 24T, and with the shaincase off, I don't see anything that would foul the chain.

Anybody know about this?

My first question is this shop a Norton shop, or a Brit Bike shop? :?
Big difference. You can easily run up to a 22 tooth.
Don't know about 23 or 24.
My suggestion would be to try a 21t., then go from there.

Akshully this guy is a Norton specialist, but he's also about 10,000 years old. I screen his advice carefully.

Thanks!

My 750 had a 24 tooth on it when I bought it but it was a non-runner so I can't say if it was over-geared (bearing in mind that it had 30mm ports and a single SU, I doubt if it pulled hard in top !)

There were certainly no clearance problems. The downside is that it is as big as a tea-plate and puts extra loading on the gearbox case and bearings. I have seen 21t recommended as the largest sensible maximum size.

Traditionally, Nortons allowed for altering gearing by supplying alternative engine sprockets which had the advantage of speeding up the gearbox and reducing torque loadings. Unfortunately, they "forgot" to continue with this practice with the triplex Commando primary drive. Most belt conversions will let you do this and I believe that I have heard of aftermarket triplex engine sprockets.

I would agree with MichaelB - try 21t and see how you find it.

I just put a 22-tooth on my 850 after a gearboox rebuild this summer and like it. About 4500 rpm at 60 mph which makes for comfortable highway cruising. Not so tall that I can't grunt it up a hill either. The bike used to have a 20-tooth on it and was revving close to 6000 rpm at highway speeds. Depending on whether you want relaxed top end cruising or a little more acceleration, I'd go for a 21-tooth or 22-tooth. 21-tooth may be more optimal for a 750.

22t was the factory fitting for european market late 850s. We were always told that the US demanded low gearing for fast standing 1/4 times whereas we were all obsessed with top speed.

The 22t was blamed for a spate of layshaft bearing failures but also co-incided I think with some dodgy bearings.

I found 22t a bit over geared on my MkIII with all the standard black chuff-box bits. I now run 21t with 750 airbox and pipes and consider it a good compromise.

I ran a 23T on my 850 for 3 years and switched to a 21T this year. I found the 21T good all around. Compared to the 23T the 21T pulls nicely from a complete stop and I didn't loose much on the highway speeds, 200 or 300 rpm difference.

I'm ordering a 21T. I tend to prefer a bit taller gearing, but this should be a subtantial improvement ofer the 19T that's on there.

I installed a 21t sprocket on my 850 Commando and feel it lets the engine relax a bit at speed while not sacrificing too much grunt down low.

Jason

so.... uh... if i have the stock 19T and cruise on the interstate for long amounts of time at 70... is it reasonably likely that i will wear down the top end to crap super quick?? nothin' like being on the highway in the middle of nowhere wondering if the engine is about to give out!