When and How to Replace a Bike Chain

Drivetrain · Beginner · ~20 min · 6 steps

Drivetrain

Chain Replacement

Replacing a chain at the right time — before it's stretched the cassette — is the single most cost-effective maintenance task on a geared bike. A chain costs £10–30. A cassette and chainring damaged by a worn chain cost 5–10× more. The process takes 15 minutes once you've done it once.

When to Replace the Chain

Use a chain wear indicator (chain checker tool) to measure stretch. Most indicators have two measurements:

0.5% stretch: Replace on 11-speed and 12-speed drivetrains — tighter tolerances mean narrower wear window.
0.75% stretch: Acceptable replacement point for 8, 9, and 10-speed systems.

Without a gauge: measure 24 links with a ruler. New chain = exactly 12 inches (304.8mm). At 12.125 inches (308mm), it's time to replace regardless of speed count.

Replace chains proactively, not reactively. If your chain is skipping under load even after a new chain, the cassette is already worn to match the old chain profile — you'll need both. Catching chain wear early avoids this cascading failure.

Buying the Right Chain

Chain width is determined by the number of sprockets on your rear cassette. A wider chain on a narrower cassette will rub. A narrower chain on a wider cassette will work but wear faster. Match chain speed-count to drivetrain exactly:

8-speed: Any standard 8-speed chain (Shimano HG, KMC Z8). Widest chains, most forgiving.
9-speed: 9-speed specific (Shimano HG-53, KMC X9). Compatible across brands.
10-speed: 10-speed specific. Cross-brand compatible (Shimano, SRAM, Campagnolo are all interchangeable).
11-speed: 11-speed specific. Road and MTB 11-speed chains are compatible. However, Campagnolo 11-speed uses a slightly different link profile — use Campagnolo or compatible chain on Campy groupsets.
12-speed: NOT cross-brand compatible. Shimano 12-speed chains are different from SRAM 12-speed chains. Match brand exactly. Eagle (SRAM MTB 12-speed) requires Eagle-specific chain.

Budget-level chains (Shimano HG40, KMC Z-series) work fine for training and commuting. Performance chains (Shimano Dura-Ace, SRAM Red, KMC X12) use hollow pins and side plates to reduce weight and can last longer with good maintenance.

Step 1 — Remove the Old Chain

Find the quick link or use a chain tool

Most modern chains (and many older ones retrofitted by the owner) have a quick link (also called a master link) — a special connecting link with two plates that snap apart without tools. To open it: hold the chain at the quick link and push the two plates toward each other while pulling them apart sideways. If you can't find a quick link, the chain was joined with a standard pin and you'll need a chain tool to push the pin out. Look for a slightly shinier or different-coloured link — that's often the quick link or the joining pin from the previous installation.

Remove the chain and assess the cassette

With the chain removed, spin the cassette and inspect the teeth. New sprocket teeth are symmetrical — square-shouldered. Worn teeth look like shark fins: asymmetrical, pointed, and hooked on the drive side. A cassette with clearly hooked teeth will skip with a new chain because the new chain won't seat into the worn tooth profile. Replace the cassette if worn.

Shimano chains must be joined with a Shimano joining pin (a slightly larger pin designed to deform into the outer link on installation). Do NOT reuse old Shimano pins — they are single-use. SRAM chains use quick links (Powerlink) designed for single use, though many riders reuse them 2–3 times with no issues in practice.

Step 2 — Size the New Chain

Use the old chain as a template (simplest method)

Lay the new chain alongside the old chain and count links to match the same length. New chains come oversized — you must shorten them. Each link pair (one inner + one outer plate set) = one link count. Most road bikes need 108–114 links; most MTB need 114–120 links depending on rear cassette size.

Measure if you don't have the old chain

Place the new chain on the large chainring and the largest rear sprocket — don't thread it through the derailleur. Pull both ends together until the chain is taut with minimal droop. Add two links (one inner + one outer) to this length and that's your cutting point. This method ensures enough length for the largest gear combination while keeping the chain short enough for the smallest. A chain that's too long sags and jams; too short and it can't reach the large ring/large sprocket combination and risks tearing the derailleur off.

Step 3 — Connect the New Chain

Thread and join

Thread the chain through the rear derailleur correctly: over the top jockey wheel, around the inside, and up through the cage to the bottom jockey wheel. The chain path forms an S-shape through the derailleur. On the front, thread through the front derailleur cage if present. Join with a new quick link or Shimano joining pin:

Quick link: Thread one plate from each end of the chain, snap together, then apply pedal pressure (or pull the chain ends apart firmly) to seat the link into its locked position. A correctly locked quick link cannot be separated by pulling the chain ends apart.
Shimano pin: Insert the tapered end of the joining pin into the chain and use a chain tool to press it through until flush. Break off the guide tip of the pin with pliers or the chain tool's breaking notch.

Step 4 — Final Check and Break-In

Check for stiff links and re-index if needed

Back-pedal and watch the chain through the rear derailleur. Any stiff link will cause a repeating jerk or jump as it passes through the jockey wheels. A stiff link is usually at the joining point. To free it: hold the chain on either side of the stiff link and flex it sideways — a short side-to-side wiggle loosens it. Alternatively, work it back and forth over your thumbs. After installing a new chain, re-index the rear derailleur — new chains have slightly different tension characteristics and may need one quarter-turn of barrel adjuster to re-sync shifting. Apply chain lube after installation.