Nobody walks into a shop asking for a 5.5 mm chain. They say "my chain is worn" and point at it. But that number is the entire answer to "will this one fit", so here it is, for the two chains people ask about most — and for two bikes in the fleet that answer it both ways.
A chain has to be wide enough inside to sit on the cog teeth and narrow enough outside to pass between cogs. As cassettes gained speeds, cogs got closer together, and chains got narrower. A 12-speed chain is built for 12-speed spacing; on an 11-speed cassette it sits low on the teeth and shifts badly. That is why our chain standard is an enum, not a number you round.
The standard is the whole story
| KMC X11 — chain standard | 11-speed (5.5 mm) |
|---|---|
| KMC X12 — chain standard | 12-speed HG+ / Eagle (5.25 mm) |
| Giant TCX Advanced Pro 2 (2023) — chain standard on file | 11-speed (5.5 mm) |
| Giant Defy Advanced 2 (2025) — chain standard on file | 12-speed HG+ / Eagle (5.25 mm) |
A quarter of a millimetre. That is the difference between a chain that runs quietly and one that skips under load. It is also why the Giant TCX Advanced Pro 2 — whose sheet lists a KMC X11 and an 11-speed GRX group — reads the X11 as "fits" and the X12 as "does not fit", while the Giant Defy Advanced 2, a 12-speed 105 bike, reads exactly the reverse. Same rule, both directions, one fact each.
A chain is the cheapest part on the bike and the one that decides whether everything else works.
KMC X11 chain, 11-speed — The 11-speed chain: fits the TCX, does not fit the Defy.
KMC X12 chain, 12-speed (Eagle / Campagnolo) — The 12-speed chain: fits the Defy, does not fit the TCX.
Drivetrain — Giant TCX Advanced Pro 2 (2023)
Giant Defy Advanced 2 (2025) — The 12-speed bike in the comparison.
Replace a chain with a quick link
- Count the links Lay the new chain next to the old one and match its length, or size it big-big plus two links if the old one was wrong.
- Thread it through Route the chain through the derailleur cage the way the old one ran — inner side of the upper pulley, outer side of the lower.
- Close the quick link Bring the two ends together with the link, then pedal backwards while holding the rear brake so the link snaps into place.
- Check it under load Shift through every cog on the stand, then ride around the block before trusting it on a hill.
- Can I use a 12-speed chain on an 11-speed cassette in a pinch?
- It will turn the wheels, and it will shift badly and wear the cassette oddly. We do not list it as a fit, because it is not one: the TCX page reads the X12 as "does not fit".
- Is a Shimano 11-speed chain interchangeable with a KMC 11-speed chain?
- For chain width, yes — both are the 11-speed standard. Shimano chains are directional; KMC chains are not. Both use a single-use quick link.
- How do I know my bike is 11-speed?
- Count the cogs on the cassette. Eleven cogs means an 11-speed chain, whatever the brand of shifter — and on a bike page in the fleet, the chain standard on file says it outright.
What to remember
- Chain width decides chain fit; the speed count is the shorthand for it.
- An 11-speed chain runs on Shimano and SRAM 11-speed cassettes alike.
- A 12-speed chain does not fit an 11-speed cassette — not badly, not at all — and the reverse is just as true.
- Quick links are single-use on 11- and 12-speed chains. Carry a spare.
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