How to Fix a BMX Chain

A dropped BMX chain is usually easy to refit: lift it over the sprocket teeth, rotate the cranks and check the tension. A broken chain needs a chain breaker to remove the damaged links and a master link or half link to rejoin it. After any repair, set the tension so the chain has a small amount of slack, about 10 to 15 mm of up and down movement in the middle. Too loose and it falls off. Too tight and it wears quickly.

Tools you need

• A chain breaker
• A spare master link (also called a connecting link) in the right size
• A spare half link, if you carry one
• A spanner or socket for the rear axle nuts
• A rag and a little chain lube

BMX chains are normally 1/2 x 1/8 inch, which suits most single speed bikes. Check yours before buying a master link.

Fixing a dropped chain

1. Stop and move out of the way. Do not keep pedalling.
2. Look for the cause. Check if the chain slipped off the sprocket or the rear cog. Look for bent teeth, a loose rear axle or a bent frame.
3. Lift the chain onto the sprocket. Lay the chain over the top of the sprocket teeth.
4. Seat the chain on the rear cog. Turn the rear wheel by hand until the chain sits on the cog teeth.
5. Rotate the cranks. Turn them gently so the chain runs smoothly around both parts.
6. Check the tension. The chain should have 10 to 15 mm of play midway. If it is saggy, tighten it. See the tension section below.
7. Wipe your hands and the chain. A dropped chain often leaves oil and dirt.

Fixing a broken chain

A broken chain usually means a bent link or a worn pin. Look at where it snapped and replace more than the single broken link if neighbouring links are damaged.

1. Remove the chain. Take the chain off the sprocket and cog.
2. Find the damage. Look for twisted plates, cracked sides or a missing pin. Mark the bad links.
3. Place the chain in the chain breaker. Seat the pin you want to push out over the breaker's pin, aligned with the tool.
4. Push the pin out. Turn the breaker handle until the pin pushes almost all the way out. Do not drive it completely free, because it is hard to refit. Leave it hanging on the plate.
5. Remove the damaged section. Cut out the bad links so you are left with healthy ends. Remove whole pairs of links where you can, so the chain stays an even length for a master link.
6. Join with a master link. Slide the master link pins through each end, add the outer plate and the clip. The closed end of the clip should face the direction the chain moves when you pedal forward.
7. Check the link moves freely. A stiff link will skip. Flex it sideways until it loosens.
8. Refit the chain and set tension.

Half links and master links

Master link. A master link is a special link with a removable plate or clip. It lets you open and close the chain by hand without a chain breaker. It is the easiest way to join a chain after repair.

Half link. Chain links come in inner and outer pairs, so a chain normally changes length in steps of one inch. A half link is a single offset link that adds one half inch step. You use it when, after cutting, the chain is slightly too long or too short to tension properly in the dropout slots. Half links are weaker than regular links, so use them only when needed and check them often.

If a chain is too long, remove links until it just reaches the slot with the rear wheel pulled back. If you cannot get the right tension, you may need a half link to take up the last bit.

Correct tension

1. Loosen the rear axle nuts. Do not remove them.
2. Slide the wheel back. Pull the wheel evenly in both dropouts until the chain is snug.
3. Check play. Push the chain up and down in the middle of the top run. You should get about 10 to 15 mm of movement (roughly half an inch).
4. Centre the wheel. Make sure it sits straight in the frame. The tyre should be an equal distance from each seat stay.
5. Tighten the nuts. Tighten them evenly, a little at a time, then recheck the tension.
6. Use chain tugs if needed. Small tensioners hold the wheel in position and stop it from sliding forward.

A chain that is too tight wears the sprocket and cog and puts stress on the hub bearings. A chain that is too loose will drop. Recheck tension after the first ride on a new chain, because it stretches slightly at first.

Why a chain keeps dropping

Cause	What to check
Too much slack	Re-tension the chain
Bent or worn sprocket	Look for hooked or bent teeth
Poor chain line	Check the sprocket and cog line up
Loose rear axle	Tighten the axle nuts
Worn chain	Measure stretch and replace
Hard landings	Check tension more often

For setup of chain line on a new build, see how to build a BMX bike from parts.

When to replace the chain

Replace the chain when:

• A 12 inch length measures about 12 1/16 inches or more.
• Links are stiff and do not flex.
• Plates are cracked or bent.
• It is rusty and lube does not help.
• It keeps skipping or dropping after you have set the tension correctly.

A worn chain damages the sprocket and cog. If you have used a chain for a long time and replace only the chain, the new one may skip on worn teeth. In that case, replace the sprocket too.

Keep it healthy

A clean, lightly oiled chain lasts far longer. Wipe it before each ride and add a small amount of lube once a week. See BMX maintenance basics.

Related

• Chain
• Sprocket
• BMX maintenance basics
• How to build a BMX bike from parts