Hi Phil,

The tooth form for a sprocket is quite involved, rapid chain wear may result if the form is not quite right. If this is for a high performance kart I think it would need to be made fairly accurately here is a link to one site with details. if it is for a low performance machine you might get away with what you propose.

Mike

Yes it can be done, I've done it myself but only for a much lower tooth count and smaller pitch chain. You work out the pcd by assuming a 70 sided polygon with each side 3/8". The angle subtended at the midde is 360/70, and the pitch circle radius is (3/8*1/2)/sin (360/70*1/2), which I make 4.1792", but check it out before cutting metal. You then file the sides until a piece of chain moves smoothly into and out of engagement. This is 140 edges, so very very tedious. Unless you're a masochist I'd buy one ready made, or find a gear cutting firm and get them to do it.

can't you find a bicycle chainwheel in the right size?

I'm not sure where you get 8.3784 from, 2* 4.1792 = 8.3584. Unless the wheel is pretty good, the chain will not mesh smoothly which could result in it coming off all the time, or snapping. Snapped roller chain whirling round next to grandson isn't an attractive prospect. Obviously you don't need to worry about the 2/10s of a thou, but 'ish' probably isn't good enough. I very much doubt that 3/8 pitch chain has 3/8" diameter rollers, depending on what the exact spec is it could be 5mm, 0.2", 6mm, 0.25"

You can get 71t sprocket and a length of chain for <£20 from this outfit

http://www.ebay.co.uk/itm/Kart-Gorilla-O-Ring-Chain-Sprocket-Offer-Best-Price-Rotax-TKM-/130962346758?var=&hash=item64289b7af2

I've no idea what pitch it is, but got to be worth a phone call.