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I know this is a highly contentious question and I am bound to get a number of answers about turning rotors at every pad replacement but this is going to be a simple replace or keep question as I don't like turned rotors.

I have had a brake pad wear all the way down to metal and it has scored the inside side of the rotor (front drivers side VX commodore if that makes a difference). Now I am replacing the pads in four days when I can get to the auto shop but am needing to work out how critical it is to replace the rotor as well. It is definately scored and can be felt and seen (compared to the smooth outside side of the rotor). Now I have been struggling to find any firm advice on this online with some sites saying the new pads will bed in and mate with the rotors even with scoring, others say rotors should be turned every pad replacement, and others still any damage at all should be turned or replaced.

My question is how much damage (ie how deep the scoring is) needs a replaced rotor (no turning replacement or keep only).

Thanks.

Additional #1 When I checked the pads to begin with I found that the rotor was sitting loose on the axle and that all that was holding it in place was the wheel and that with the wheel being removed the rotor could move a fair bit on the wheel bolts and was only being retained at that point by the brake caliper. Is this normal for the VX Commodore or should the rotor be attached through additional screws or bolts.

Thanks

  • "the rotor was sitting loose on the axle and that all that was holding it in place was the wheel" - That's OK. It won't come off while driving. If it does, the wheel has also fallen off, so the rotor is the least of your problems at that point :) – PeteCon Mar 3 '17 at 16:16
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Yes, if they have been scored such that you can feel or see it you should replace it.

I'm not sure if that model has a retaining screw for the discs, but you don't need to fit it. It is only there to make work easier when the wheel is off. After that, it's redundant. You can use a wheel nut without the wheel to do hold it on when you're working.

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I agree with @DizzyFool here, but wanted to add some context.

Overall, since you don't like to cut the old rotors, if you can feel the ridges your only option is to replace the rotors. The reason why you want to replace the rotors is so they will work optimally.

When you put new pads on a worn rotor, the braking system will work, but only sub-par. For the braking system to work at its best, the mating surface on the rotor needs to be as pristine as possible. Having the ridges there does two things which aren't good. First, you don't get all of the pad to rotor contact which occurs on with freshly machined rotors. This only gives you a portion of the braking ability which the brakes should be able to apply. Secondly, the pads will ultimately "break-in" and get worn down to mate better with the ridges on the rotor, which isn't terrible, as this will provide a little better braking ability. The bad thing, though, is when it happens, there goes a portion of the friction material which would not have been used up otherwise. This will require you to replace the pads sooner than you normally would have.

All-in-all, you could use the old rotors with ridges, but it's pretty compelling to replace them with new ones due to what happens if you don't. You can either replace them and have great working brakes with long lasting pads, or find you'll need to replace the pads again in a relatively short time. Having freshly machined rotors only makes good sense.

  • "Secondly, the pads will ultimately "break-in" and get worn down to mate better with the ridges on the rotor, which isn't terrible, as this will provide a little better braking ability. The bad thing, though, is when it happens, there goes a portion of the friction material which would not have been used up otherwise." If the grooves are 0.1 mm deep, wouldn't this only matter about 0.1mm/12mm when new ~= 0.8%? Replacing pads 1% more frequently wouldn't be noticeable. – Ives Mar 3 '17 at 18:14
  • @Ives - Even so, it would be more than it would have if the rotor face were completely flat. The point is still valid. Obviously smaller ridges would equate to smaller amounts of imminent wear, but it's still more wear. The best choice is to have a machined face to start with. – Pᴀᴜʟsᴛᴇʀ2 Mar 3 '17 at 20:38
  • I agree of course. Seeing as how new pads are flat, your rotor should also be flat for best contact. – Ives Mar 4 '17 at 15:50

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