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In all my years of working on bikes, this is a first. Bought a new 9 speed KMC chain and 9 speed cassette to replace a worn 9 speed drivetrain recently (single chain ring). Went to put everything on tonight, except the new chain is way too short -- even on the smallest cog the derailleur is so stretched I'm afraid turning the cranks will rip it off the hanger.

Comparing the new chain to the old chain (also a 9 speed chain, in case it isn't clear) shows that the new chain is about six inches shorter. I don't know how many links are in the old chain, but the new chain supposedly has 116 links and the highest link chain I can find on the internet is 120. I've never had so much difficulty overhauling a 9 speed drivetrain before.

Can anyone explain what the hell is going on here? I've got pictures on my phone I can include if the description doesn't make sense.

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    The length of chain in inches or millimetres isn't that important - what matters is the number of links has to be identical to the old chain. Photos might help clarify the cause, so yes please add your pictures.
    – Criggie ♦
    Oct 5 '19 at 6:28
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    The replacement chain is too short!
    – Daniel R Hicks
    Oct 6 '19 at 19:21
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    Two same # speed chains can differ in the number of links / chain length. The # speed concerns chain width and its ability to bend sideways. When looking for a replacement you also need to consider how many links you need depending on your frame size and cogs and chainrings sizes.
    – Robert Lee
    Oct 6 '19 at 21:08
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    The ''speed' description of a chain is not a misnomer. 8, 9, 10, 11 speed chains differ mostly in the external width of the chain which gets progressively narrower (the internal width between the inner plates is constant at 2.4mm.) The external width decreases because the separation of sprockets gets narrower as the number of sprockets goes up. All bicycle chain has a pitch of 1/2".
    – Argenti Apparatus
    Oct 7 '19 at 2:00
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    Weird to specify the distance between rollers in thousandths, since it's always 1/2" or 12.7mm
    – ojs
    Oct 7 '19 at 7:14
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Pretty simple - the new chain is too short. Maybe you bought a chain with too few links, maybe it's a production error.

The pitch (distance between pin centers) of bicycle chain is 1/2", so it's easy to count links by measuring.

Chain length required is determined by chainstay length and largest chainring and sprocket size. Usually you get a chain a few links too long and cut it to size and re-join. When replacing a chain you can use the old chain to size the new one. If the old chain is lost chain length can be determined by fitting to the bike. Park Tool has a good video on how to do that.

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    As an example: a new chain with 116 links was too short for my 1× conversion — the position on the biggest rear cog was stretching the derailleur too much. A chain with 118 links was just fine. If a setup has a huge front chainring (40t), the rear cassette has similarly huge biggest cog (now one can have 50t for 9-speed (sic!)), and the chainstay length is big, one may need first to connect two regular chains, and then cut it to make a chunk long enough. Or maybe get a custom length chain, if that's a thing: tandem bicycles do need to have even longer chains.
    – Grigory Rechistov
    Oct 7 '19 at 6:30

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