Replacing Front Disc Brake Rotor (PX 125, 150 & 200)

1. Remove the front wheel rim and tyre.

3. Remove the wheel hub from the stub axle.

The threadlocked rotor bolts can very difficult to remove, and instead of using an Allen key, it may be easier to use a ratchet wrench with a HX4 socket attachment as shown in the picture below.

The Haynes manual has this to say about inspecting the old rotor for wear:

Inspect the surface of the brake disc for score marks and other damage. Light scratches are normal after use and won't affect brake operation, but deep grooves and heavy score marks will reduce braking efficiency and accelerate pad wear. If a disc is badly grooved it must be machined by a brake specialist [which is probably not worth it considering the low cost of replacement] or renewed. There is no service limit for the brake disc, but the extent of the disc wear will be apparent from the size of the lip around the unworn outer edge.

In the picture above, while the side shown of the rotor was only relatively moderately worn, the scoring and damage to the other side of the rotor (the pads were worn to the metal) are clearly seen in the pictures below:

5. Install the new rotor. Make sure that the rotor bolts are fastened with threadlock. In order to avoid warping the rotor, when tightening the rotor bolts, make sure pressure is applied gradually and evenly around the rotor, moving from one bolt to the next in a stellate pattern. Install the wheel hub back onto fork (PX front wheel hub nut torque wrench setting 54.3-65.1 lbf ft/ P125 150X and P200 E 43.4-72.3 lbf ft). Clean all grease off the new rotor with brake cleaning fluid.

6. It will probably be necessary to change the brake pads at the same time. For this procedure, see Changing front brake disc pads

8. Give the rotor one more clean! And then attach wheel rim and tyre.