Motocross chain and sprockets: when to replace them and how to look after them

01How long a motocross chain kit lasts

A motocross chain wears 3-5× faster than a road one: sand and dirt thrown up at it, brutal loading under acceleration and deceleration, and often not enough lube.

02The signs it needs replacing

The chain

• Tight spots: spin the rear wheel in the air and watch — if the chain binds at one point, it's finished
• Stretch: measure 20 links. New = 635 mm (pitch 12.7 mm × 50 = 635). Over 640 mm (+1 %) → worn. Over 645 mm → dangerous.
• Side play: waggle a link sideways. More than 2 mm means the pins are worn

The front and rear sprockets

• Hooked teeth (wave or shark-fin shape) → worn, replace them
• Pointed teeth instead of rounded → worn
• Golden rule: you ALWAYS replace chain + front sprocket + rear sprocket together. A new chain on a worn sprocket is destroyed in 10 hours.

03Maintenance to make it last

After every motocross ride

1. Low-pressure hose (keep the pressure washer off the chain) 2. Chain cleaner if it's really caked (WD-40 Specialist, Motul Chain Clean) 3. Lube with O-ring chain grease (Motul C2, Bel-Ray Blue Tac)

The correct tension

• Bike on a stand, rear wheel off the ground
• Halfway along the lower run (between the two sprockets): 40-50 mm of slack (3-4 fingers)
• Too tight → premature wear plus a broken output shaft
• Too loose → the chain jumps, chain and sprockets wear, and it's dangerous

04Chain kits worth buying

Sources: 24mx.fr and crossmoto.fr catalogues

05O-ring or not?

• Without O-rings: cheaper, lighter, less friction. Ideal for pure motocross (it's not going to last long either way).
• O-ring / X-ring: more expensive, longer-lasting, less maintenance. Ideal for enduro and trail.

The chain doesn't lie: lube it after every ride and keep the tension right, and it lasts twice as long.