Front sprocket

The front sprocket is the small toothed wheel on the end of the gearbox output shaft, on the left side of the engine behind a small cover. It is held by a bolt and plate or a nut and lock washer.

What it does

The gearbox turns the front sprocket, which pulls the chain. Because it is small, each tooth does more work than a tooth on the rear sprocket, so it often wears faster. The number of teeth sets the gearing: one tooth fewer gives stronger pulling but lower top speed and more fuel use.

Signs it is worn or failing

• Teeth hooked, pointed or thin: Sprocket teeth hooked or broken.
• Chain noise or jumping teeth: Chain noise, Chain jumps.
• Oil behind the sprocket means the shaft seal, not the sprocket: Oil leaking behind the front sprocket.

When to replace it

Replace when teeth are hooked or bent, or about a quarter of the tooth is worn away. Always replace it with the chain and rear sprocket as a set.

What to match when buying

• Chain size (for example 428), tooth count and spline pattern of the output shaft.
• Offset (how far the teeth sit from the mounting face) must match, or the chain runs out of line.
• Match make, model, year and engine code; the tooth count is usually stamped on the face.
• Changing the tooth count changes the speedometer reading on bikes that measure speed at the gearbox.
• Good sprockets have hardened, evenly cut teeth. Soft cheap sprockets wear in weeks.

Fitting notes

• Use a new lock washer or plate if bent. Tighten the bolt to the value in your bike's manual.
• Look at the output shaft seal while the sprocket is off.
• See Inspect sprockets and Replace the chain and sprockets (workshop).
• A smaller front sprocket (or larger rear) gives stronger pulling and lower speed. A larger front sprocket (or smaller rear) lowers engine revs at cruising speed. Changing the front by one tooth has a bigger effect than changing the rear by one.
• Do not undo the retainer without the chain on, or with the rear brake held by a helper, since the sprocket will turn.

Reference (K-Light 125)