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Car is E46 BMW.

One of my front brakes were squealing on light brake pressure, so I got the rotors machined. As the pads had a lot of material left (9mm), I thought I'd reuse them. However, after fitting everything together and going for a test drive, the car shudders from the front right brake on hard braking.

After examining the used pads, the outer brake pad was uneven along the long edge. I will be replacing the brake pads to match the newly machined rotors.

What should I be doing / checking to ensure that these pads don't become unevenly worn? I have checked the caliper guide pins and they're moving freely, but not much else. Does the caliper piston seal need to be uniform around the piston?

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  • You can reuse brake pads, but you have to use sand paper on a very Flat surface and sand the glaze off and get the surface even before putting them back in service.
    – Moab
    Commented Feb 10, 2020 at 13:49

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There isn’t really anything else you can do.

New rotors (discs) and new pads will give the best success. Scratches on the pad can easily occur from a small amount of rust on the rotor, or even a particle of dirt getting between pad and rotor. Once the pad has been scratched it won’t press against the rotor where the scratch is allowing rust to build up at that point on the rotor. That rust will scratch the pad further allowing more rust to build up and hence the pad becomes uneven. Using that pad on a new rotor will immediately allow rust to start on the new rotor where the pad doesn’t touch.

I wouldn’t expect uneven pads to cause shuddering. I would expect pulling to one side if one brake works better than the other due to uneven pad contact. It is more likely that one of your rotors has a very small amount of runout i.e. it has a slight wobble around its circumference due to not being machined correctly. You could attach a dial gauge against the rotor while fitted to the hub to measure the run out. Also check that the rotor is fitted flat against the hub and that there is nothing trapped between them.

If the machining is not correct, the rotor will push the piston in and out as it is rotating, causing uneven braking and a shuddering.

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  • I applied anti-seize to the rotor hub with a paint brush and a few brush hairs got stuck to the hub - would this be enough to cause it to not sit properly?
    – tgun926
    Commented Feb 10, 2020 at 10:15
  • Possibly. I would remove any debris and only put a very thin coat of anti-seize. If the rotor isn’t sitting flat against the hub, you could have a wobble.
    – HandyHowie
    Commented Feb 10, 2020 at 13:48
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One common cause of uneven brake pad wear is if the moving portion of the calipers do not move smoothly and freely on the guide pins (attached to the fixed portion of the calipers). BMW calipers are rebuildable and you can buy new guide pins and seals, as well as grease for them, separately. You could also just disassemble the calipers, clean the guide pins (and possibly sand/polish if necessary) and regrease them. As long as they're not too worn, you can get away without replacing them.

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