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Haynes Workshop: YZF-R125 chain/sprocket change

Firstly, support the motorcycle so that the rear wheel is off the ground and tie the front brake on to prevent the bike from rolling forward.

Remove the front sprocket cover (photo 1). Locate the chain’s joining link in a suitable position to work on by rotating the back wheel. Release the clip from the master link. Remove the sideplate and the O-rings. Withdraw the master link from the inside and remove the O-rings (photo 2).

Unscrew the front sprocket retainer plate bolts (photo 3).Turn the sprocket retainer plate to unlock it from the splines. Slide the sprocket off the gearbox shaft. Remove the chain from the bike, noting its routing through the swingarm.

Remove the rear wheel. Unscrew the nuts securing the sprocket to the wheel hub. Use a degreasing fluid to clean the oily dirt and old chain lube from the back of the engine, inside of the sprocket cover, swingarm, chain guard and rear wheel hub..

Before fitting the new chain and sprockets, check that they are the correct size.The chain size is scribed on the master link sideplate and the sprocket; number of teeth are marked on each sprocket’s outer face (i.e. 14T and 48T in the case of the YZF). Slide the new front sprocket on to the gearbox shaft with its marked side facing out. Fit the retainer plate, then turn it in the groove so it is locked in the splines and the bolt holes align. Install the retainer plate bolts and tighten them.

Before fitting the new rear sprocket, check the condition of the sprocket studs and nuts and renew them all if any are damaged.The studs can be removed using a stud tool, or by threading two nuts on to each stud and locking them together, then unscrewing the stud using the bottom nut.

Apply a suitable thread locking compound to the threads of the new studs and tighten them into the sprocket coupling. Fit the new rear sprocket on to the wheel hub with its stamped mark facing out. Fit the nuts and tighten them evenly and in a crisscross sequence. Install the rear wheel.

Slacken the chain adjuster locknut on each side of the swingarm and turn each adjuster bolt inwards by a few turns (clockwise) (photo 4). Route the new chain around the sprockets and through the swingarm, leaving the ends in the middle of the lower run. Fit a new O-ring on to each of the master link’s pins. Fit the master link through each end of the chain.

Fit a new O-ring on to the outer end of each pin. Fit the sideplate with its mark facing out, then squeeze the plate using grips and a socket to compress the O-rings to expose the grooves for the clip. Locate the clip and slide it across the groove in each master link pin, making sure the closed end points in the direction of chain rotation (photo 5). Make sure the clip has located in the grooves and around the end of each pin.

Set the bike on its sidestand. Using a ruler, measure the total up and down chain slack midway between the sprockets on the bottom run. For theYZF this slack should measure between 30 to 40mm (photo 6).

Turn the chain adjuster bolts anti-clockwise to take up excess slack, making sure the front edge of each adjustment marker aligns with the same index line on the swingarm – failure to do so will result in poor wheel alignment.Tighten the adjuster bolt locknuts. Spray the chain with an aerosol chain lube.

This workshop routine is taken from the Yamaha YZF-R125 ’08 to ’11 manual (Haynes 5543). Visit www.haynes.co.ukwww.haynes.co.uk for more information