Forum

[oldnick]

What’s the current view on changing the gearbox sprocket from 17 to 18 teeth on a 2017 B5 Euro4 EFI? I feel it could pull 60mph with less revs for a smoother cruise. The job looks OK except removing the 46mm nut – is that usually a problem?

[Revband]

You would do better going up two teeth, or you may need a chain with a half link?. You will probably need a new chain as well. One tooth is approx 4% = 2.2 mph at 50 not much difference.

[Rattlebattle]

It’s a matter of preference really. A one tooth increase on a 17 tooth sprocket is actually just under 6%. If you ride two up or live in a hilly area it wouldn’t be a good move but I am pleased with a one tooth increase on my C5. No need for a half link; the standard chain was fine on my C5. Don’t bother buying the sprocket nut box spanner, a 46mm socket will do fine. I would order a new tab washer as the original will probably get damaged removing it. The main risk is damaging or displacing the o ring. Use a dab of grease to keep it in place when refitting the outer cover. Hope this helps.

[Revband]

Oops, approx 7.5% 3.75mph @ 50.

[Rattlebattle]

Forgot to say: two teeth increase needs a bit of filing to give clearance. Not recommended as it will kill what little power there is and it probably won’t rev out in top gear, on a standard engine.

[oldnick]

Thanks for the advice. Did you go from 18 to 19 teeth on the C5 Rattlebattle? It's a bit confusing as H lists the latest C5 as having a 17 teeth sprocket but the earlier ones with 18 teeth. I was thinking of a new chain as the original fit seems to have a dubious reputation - it certainly seems to stretch quite a lot. In which case, would you go for a 102 pitch chain normally fitted to the Classic with 18 teeth sprocket (the B5 with 17 teeth has a 101 pitch chain). Your views much appreciated.

[oldnick]

I may have learned a little more about the gearing for the B5 Euro4 with rear discs. It now has an 18" rear wheel (previous was 19") yet, according to Mr. H, still retains the 17 teeth sprocket which may be part of the reason why it feels under-geared to me.

[ric]

The main advantage of changing from a 17 to an 18 on the front is you'll no longer be reliant on the half link [br]
I did this to my 2015 B5 and don't intend going back to a 17.

[Rattlebattle]

Sorry for the delayed response. Yes, I went from an 18 to a 19 on my C5. I had forgotten that B5s have a one tooth smaller front sprocket to compensate for the 19 inch wheels, or they did in 2015 when I bought my C5. My comment about going up two sizes only applies to the C5 ie a 20 tooth sprocket. A little relieving with a file is needed; anyway that would be too tall IMHO. I took the opportunity to replace the terrible original chain with a Reynolds. Same number of links. I can’t believe how much more smoothly the rear wheel now rotated. The original had more tight spots than a drunken leopard. FWIW I replaced my primary chain with a Reynolds too after another post about someone who found two half rollers in his discarded oil). If you ever owned a mini you might already have a suitable size imperial socket or spanner for removing the gearbox sprocket. I can’t remember what size it is in imperial or what it fits on a mini. Maybe an engineer used to doing conversions can chip in. FWIW it’s the same size as the rear wheel nut on a Triumph Sprint too...

[oldnick]

Thanks Rattlebattle. It seems odd that the B5 Euro IV still has a 17 tooth sprocket (H has stripped one recently and it is 17) since it now has an 18" rear wheel the same as the C5. I think I will just go up to 18 teeth although 19 would make it the same as yours.