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I just got a 16 tooth and tried to put on the old chain, BUT while it fits on the pedal cog (46 tooth), it does not on the rear cog. How can I solve this? a different cog in the back or a different chain? (on the rear cog, the chain has bigger "holes" than it should, so it doesnt fit...) Thank you so much for any advice...!

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Ok, so i figure the spaces are to small on the rear cog, compared to the front one (a difference of about 3mm) so should I get new chain or new a different rear cog? Thank you – Matej Dlabal Feb 6 '14 at 7:49
Check chain length. Ten chain links should be ten inch on a new chain. If they are more than 1/10" off, you'll need a new chain and probably new front chain ring, if the teeth are worn. – arne Feb 6 '14 at 7:55

You might have bought the wrong width chain. For road bikes/fixie there are two possibilities. These are the width of the chain. 3/32" and 1/8"

a 3/32" chain will not fit on a 1/8" cog but a 1/8" chain will fit on a 3/32" cog (but with extra space around the teeth).

If it is the length of the chain you are talking about then get a new chain. but the pedal cog might also need to be replaced.

More economic route might be buying a used rear cog (be warned this might be unsafe if you end up riding a lot wear will be faster)

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Buying worn parts to match other worn parts is a reeeeeeally bad idea. New chain: Wippermann 1Z1, Sram PC1 or KMC S1 are three good 1/8th options, less than 10GBP each. New chainring: 1-speed (no ramps) for 5-bolt 130mm about 25GBP. – Emyr Feb 6 '14 at 16:57

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