Ahoy there chaps and chapettes,
I got a new rear wheel today, a proper single speed but now the chain i was using (the same one from the original 10 speed minus a few links) doesnt fit. Because im new to all this im guessing that now that my rear cog has 16 teeth instead of an odd number that that my old wheel had, ill need a new sized chain. Can someone tell me what i need. With i also have to change my front/pedal chain ring aswell. Can someone kindly tell me what i need. Ill have a google around to see if i can work it out myself, but i thought id also ask you knowledgeable peeps on here.
thanks in advance
trent
Sounds like your old rear cog is 3/32" thick, like most geared bike cogs, and your new one is 1/8" (i.e. 4/32") thick, like most track cogs. If so, you’ll need to get a new chain with an internal width of 1/8". If your chainring is still 3/32", I believe you can keep using it; I’m told that 3/32" cogs and rings work fine with a 1/8" chain, although I’ve never tried it myself.
See this page on the late Sheldon’s site:
http://sheldonbrown.com/fixed-conversion.html (the bit titled “Drive Train”).
And read the rest of his site while you’re there, it’s all gold! 
@Hakucho has pretty much said it all but I thought I’d add a bit more. Regardless of the number of teeth on your cog or chainring the distance (pitch) between the teeth will always stay the same at 1/2". This is pretty much an industry standard these days, although some have tried to fuck around with it in the past with mixed results.
what they said…
But what do you mean by “doesn’t fit”? Do you mean the chain won’t drop on the teeth properly or do you mean the chain is too short or too long?
If you’re changing cranks and/or chainrings anyway, you should sort that out first and then get a chain to fit your teeth size and chosen gear ratio.
- work out the gear ratio you want
(with a 16T cog you’ll probably want a chainring around 42T-44T) - get a chainring that matches the width of your 16T cog
- get a new chain and you’ll have to either get a shop to fit it, or get a chainbreaker and do it yourself