Final drive ratio & chain length

[clanger9]

Yep, it's me again...

Now that the bike is in bits, I noticed the gearbox output sprocket is 17t. I have a new 18t one here, so I presume it would be A Good Idea to fit it?
If I change the sprocket, will I need a longer chain? 123 links? The current one is 122 links.

[Rob Harknett]

What does your parts list say?? Now don't say you have not got one , it's a must have. If you look you would had discovered the answers. You do not say what rear wheel sprocket you have. Now look in the parts list for your bike, I don't have one, but I do have a 1960 AJS book. Unless your bike's year is different from 1960, I see, you should have a 125 link chain and a 19T gearbox box 17T is for a CS model with 131 link chain.. This seems to suggest you may have a non standard G2 rear wheel sprocket??? I don't think your question about sprocket and chain can be answered, if your bike parts are the same as 1960. It would be best if you looked at the parts on your bike. Compare with the parts list what you should have. Then decide what to go for. Getting it correct I think the best option, then you will also have correct gearing. 123 link chain for a 350. Your chain is 122 link, so may had been altered to suit a miss match of sprockets.

[Janet]

Should you change it? That depends on why you want to.
The scramble model had17T gearbox sprocket but it had a huge back sprocket. The CSR had 18T gearbox and 54T rear sprocket. The standard G2 had, I think 19T and 55T rear.

[clanger9]

Hi Rob,

The parts list isn't always a huge help for me as the bike has been modified a lot. The final drive parts are particularly confusing as there seem to be many variants!
My bike is technically a 1966, but I've found to my cost that I have to check everything as the parts don't always match up.

It's OK, if the parts list doesn't have the answer then it's back to first principles to puzzle out the correct chain length...

[clanger9]

Should you change it? That depends on why you want to.
The scramble model had17T gearbox sprocket but it had a huge back sprocket. The CSR had 18T gearbox and 54T rear sprocket. The standard G2 had, I think 19T and 55T rear.

Thanks Janet. I think I read elsewhere that lightweights are a bit under-geared. If so (and the fact that there's a new 18t one here in my box o' bits), I thought it might be a good idea to swap it while I've got the gearbox off.

I don't know what the gear ratios are like because I've still not actually ridden my bike. One day, one day...

[clanger9]

Ah, I think I figured it out:

According to the workshop manual 350 Lightweights and 250 CSRs should have an 18t front, 55t rear and a 123 link chain.
My bike currently has a 17t front, 55t rear and 122 link chain. Which isn't correct.

That's settled then. I'll fit the new 18t front and order a new 123 link chain. Then it'll be back to standard spec.

PS: I guess you can't get replacement rear sprockets for Lightweights? Oh well, the one that's on is a bit worn, but it'll have to do.

[Rob Harknett]

Ah, I think I figured it out:

According to the workshop manual 350 Lightweights and 250 CSRs should have an 18t front, 55t rear and a 123 link chain.
My bike currently has a 17t front, 55t rear and 122 link chain. Which isn't correct.

That's settled then. I'll fit the new 18t front and order a new 123 link chain. Then it'll be back to standard spec.

PS: I guess you can't get replacement rear sprockets for Lightweights? Oh well, the one that's on is a bit worn, but it'll have to do.

That's settled then, assume the rear sprocket is standard. Gearing may not be correct, if that 17T sprocket was actually matched to a CS gearbox.
If the rear sprocket is just a bit worn, after 50 years, there's probably still a bit more life left in it. So no need to change it, unless you intend to ride it for another 50 years. I built a pre war 36 bike from worn out parts. Or were they?? After 30 years they are still there, I'm sure, still with enough life left to last me out. I have 1958 G2, 63 14 CSR & 66 G2CSR. Guess I will never need the pair of complete spare wheels I have. But they are really only saved from loads of spares, as my 63 bike had non standard wheels fitted.

[Ozmadman]

Ah, I think I figured it out:

According to the workshop manual 350 Lightweights and 250 CSRs should have an 18t front, 55t rear and a 123 link chain.
My bike currently has a 17t front, 55t rear and 122 link chain. Which isn't correct.

That's settled then. I'll fit the new 18t front and order a new 123 link chain. Then it'll be back to standard spec.

PS: I guess you can't get replacement rear sprockets for Lightweights? Oh well, the one that's on is a bit worn, but it'll have to do.

And it will still be under geared in my opinion!! ALL lightweight gearbox internal ratios are the same and interchangeable apart from the main shafts where there were two different types with each type having a mod hence the discussion on another thread about clutch centres..All the ratio changing was done with the front and rear sprockets. My 350 had 18/55 gearing and 1st gear was a waste of time and it seemed to be revving too hard at around 50mph. I fitting a 19 front and that was a bit better but eventually I fitted a 20 front and for me that is perfect. Depends on what you want, if you are plodding around town in traffic, live in Cornwall with 1:4 hills or weigh 20 stone then keep it 18/55. If you want to go on longer runs on the open road and maintain 55/60 mph and/or you are a "lightweight" 10.5 stone weakling like me then definitely change it to 20/55 (30mph will be the absolute minimum in top with this set up) only my opinion though.

No the rear sprocket is not available as yet,, a possible new item though that may get manufactured by the club if there is enough pre-paid demand???

[ajscomboman]

4 shafts Paul, 2 for the earlier boxes and 2 for the later.

[Janet]

I do town traffic (although nothing like in Southend ) and live in Yorkshire where 1 in 4 hills are common and 1 in 3 are not unheard of, but I also go on longer runs on open roads ( even occasionally on motorways ). I was happy with the standard 1963/64 CSR 54 tooth set up and only changed my back sprocket this year to a used 55 tooth one after I'd exhausted every other idea of what was wrong with my bike. The extra tooth means the engine has to work a little harder to achieve higher speeds but I'm aware of it so restrict my right hand slightly. It should pull better up hills although those people who have followed me on runs will know that my bike would romp up hills anyway so that 's a difference that I don't notice. Load-wise on the little bike, well, I'm not 20 stone but I'm no lightweight.

I also have a standard Honda 400/4 so perhaps I'm just used to listening to high revving engines that I need to play the gearbox to produce its optimum performance. For me, adjusting to the different characteristics of each bike is part of the fun of riding a motorcycle. Obviously other people have different requirements and change their bikes to suit them.

Regarding the availability of rear sprockets, please contact the spares scheme and ask them if they'll look into having some made. If enough people say they want one, we may get them. I'd prefer to get a 54 tooth one, although I'd be prepared to accept the standard G2 sprocket with 55 teeth in the absence of anything else.