Can anyone tell me where I can get a 15 tooth engine sprocket for my B25T engine. I believe it would have the same spline size as for a B40 or 441 BSA. I believe most people use an 18 tooth sprocket (which I currently have) but I know that with a half link in the duplex chain a 15 tooth sprocket will fit without the chain being too loose. Although not as strong as a full link, does anyone know of any problem using half links? I have already reduced the gearbox sprocket as small as I can and even with a 60 tooth rear wheel sprocket the gearing is barely low enough, so since I can't lower the gearing within the gearbox I prefer to get a smaller engine rather than put a large rear sprocket on the back wheel.

have a look here

http://www.trialsbits.co.uk/index.php?cPath=85_182

Thanks for the suggestion, unfortunately they only have the 18 tooth sprocket. The more I talk to people about this question the more I believe it hasn't been done before. Hopefully someone can prove me wrong

Try these people

canddautos@eidosnet.co.uk

Thanks for the suggestion. I have sent two e-mails to them but they don't seem to be getting through. I will keep trying, maybe their system is down for some reason

Thanks for the suggestion. I have sent two e-mails to them but they don't seem to be getting through. I will keep trying, maybe their system is down for some reason

Just checked their web site, new address

canddautos@gmx.co.uk

Thanks, that seems to work. I did get a reply from Burton Bike Bits, unfortunately their answer was that only an 18 tooth was available. Looks like I will be making one. I may have to invest in some more tools, the current collection of just a drill, hacksaw, grinder, welder, and files is making bike building too ,more challenging than it need be.

Thats a massive jump, from 18 down to 15 tooth. Roughly equivalent to going from 60 tooth up to 72 on the rear. (I think!!)

Try to get a WD B40 gearbox cluster. Its wide ratio, giving low 1 2 and 3, but a higher (normal) 4th. You can keep standardish sprockets and still have low 1st etc.

Your math is correct Matchless man, the 15 tooth allows me to reduce the rear wheel from 60 to 50 teeth (60 teeth looks ugly). The gear box has a 12 tooth sprocket which I could change to 11 but that won't help much. In Canada (i.e. Ontario) the vintage trials sections are fairly tight since the bikes are nearly all more modern twin shocks with several people riding monoshocks, hence the need to lower the gearing probably more than would typically be done in the UK. You would think that with a country this large that they could design a few long sections, but no. I normally ride a 500 triumph and figured I would try a "light weight' 250 BSA. I just finished making a 15 tooth engine sprocket and tried it out at a recent trial and it seems O.K. when used in conjunction with the 50 tooth on the rear wheel. I made it by grinding off the teeth from an old sprocket to form a hub and then welding on t "weld on" sprockets, the end result wasn't too bad.

Will a WD B40 set of gears fit in the B25T cases? and where on earth could I get a set?, it would be a preferable solution.

You are barking up the wrong tree!!! Try and get hold of a C15G or B25 box and fit a C15S bottom gear...I have done this Mod many times and it works well with an 18T Eng Sprocket for Trials...If you are going scrambling then a 23t front.

Try talon gears they might go down to 15 teeth

Hi Smiley, thanks for your comment. I am now up the tree having made a sprocket and used it in a trial a couple of weeks ago. It seems to work fine and is no problem starting since I use a valve lifter. I would prefer to lower the gearing within the gearbox as you suggest since the current gear spacings are not great. I didn't know there were lower gears available that would fit the B25T cases. You suggest using C15S bottom gear but the Roy Bacon book I have on BSA singles says that the C15S bottom gear is 2.667:1 while the B25T is 2.652:1. This wouldn't change things much, am I missing something? your help would be appreciated. Also there are virtually no C15 bikes on this side of the pond so can you suggest a possible source other than E Bay?

Email me at kobixion@btinternet.com I will see what i have ( good books but don't believe all you read )