I replaced a 18T sprocket of a bike with a Shimano Alfine 11 internal gearhub with a 22T sprocket (G-7C20 SPROCKET WHEEL 22T). Hence, I need a longer chain. I bought a KMC singlespeed "wide" chain. However, I then noticed, that the old chain was a KMC singlespeed "narrow". The crank set is a Shimano FC-S501 (170). Obviously, the new chain does not sit as tight as the old one. Is that a problem?

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The narrow size has a 3/32" inner width and the wider is 1/8". Chainrings and cogs for single gear applications (3-speed, BMX etc) will usually be sized for one or the other, meaning they will neatly fill out the gap.

There is no functional issue with running 1/8" chains on 3/32" cogs and chainrings on a single ring, single cog bike. The chain will have some free play on the teeth when not under tension, which you might find detracts from the quietness of the drivetrain in some situations. Also one might be able to find some examples where a 1/8" doesn't fit alongside a chainring guard if present.

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