Replace the chain and sprockets

The chain, front sprocket and rear sprocket wear together and are replaced as a set. A new chain on worn sprockets, or the other way round, wears out quickly. This page is an assessment and hand-off: it tells you how to identify the right parts and what a good workshop does. Joining a chain correctly needs the right tool and the chain maker's instructions, so it is treated as workshop work.

Safety

• Do not ride on a chain with cracked plates, missing rollers or broken sprocket teeth. A snapped chain can lock the rear wheel or break the engine case.
• Cutting an old chain with a grinder throws sparks. Keep well away from the fuel tank and wear eye protection.
• The rear axle nut must be tightened to the correct torque after the wheel is refitted.

Identify the parts (rider or home)

1. Read the size stamped on the chain side plates, for example 428 or 520. K-Light 125: 428HO-1, 134 links.
2. Count the links, or read the number on the old packet. K-Light 125: 134 links.
3. Read the tooth count stamped on each sprocket. K-Light 125 rear sprocket: 41 teeth. Fit the same counts as before; changing them changes the gearing and the speedometer reading.
4. Note the joining method on the old chain (clip link or riveted link) and whether it is a sealed (O-ring) chain.

What the workshop will do

1. Remove the front sprocket cover and the front sprocket (bolt and plate or circlip). K-Light 125 front (drive) sprocket bolt: 10 to 16 N·m.
2. Remove the rear wheel. On a chain with no joining link, the swingarm must come out to fit it; the K-Light manual removes the swingarm.
3. Remove the rear sprocket nuts in a criss-cross order, then the circlip, washer and sprocket from the hub carrier.
4. While apart, check the cush drive rubbers (Check the cush drive rubbers), the chain slider and the wheel bearings.
5. Fit the new sprockets. K-Light 125 rear sprocket nuts: 45 to 50 N·m, tightened in two or three passes in a criss-cross order.
6. Fit the new chain. Join it with the link supplied, following the chain maker's instructions for clip direction or rivet flaring and its measurements.
7. Set slack and alignment (Check and adjust chain slack), tighten the axle nut to the confirmed torque and lubricate.

What to ask for

• Chain and both sprockets replaced together, with the same sizes and tooth counts.
• Old parts back, so you can see the wear.
• The joining link fitted with the proper tool, and the axle torque value used.

Do not attempt at home

• Do not rivet a joining link with a hammer and punch. A badly flared rivet lets the link come apart.
• Do not over-tighten a new chain to "allow for stretch". Set it to the specified slack.

If it did not fix the problem

• New chain runs on one side of the sprocket or rubs: Chain rubs on the guard, slider or swingarm.
• Clunk when opening and closing the throttle remains: Clunk or snatch from the rear wheel.

Reference values (K-Light 125)