How to decide when brake disks need replacing?

clonagam

I have replaced brake pads several times but have yet to do the same with the disks. Frankly, the safety implications scared me off. However, recently I have been looking at some of the videos and it seems it could be within my competence.

a.How does one know when to replace disks?
b.If it comes down to measuring the thickness of the remaining disk what instrument can I use to take this measurement?
c.And what about score marks, how deep must they be to merit replacement of the disk?

(The car is a 2005 Honda HRV)

Any advice would be appreciated.

Mech1

Just make it easy on yourself, if the discs are cheap enough then put new discs with every 3rd set of pads.

If discs are crazy money, measure them.

paddysdream

Often wondered about this.Either I am very lucky or very easy on discs as have only replaced one disc in over 20 years motoring and that was due to a crack.
For example present vehicle has 260k on it and I bought it with 60k on it and discs are original.Same with wife's previous car.She bought it in 07 with 40k and sold it this year with 250k and never a disc changed.
Probably 3 or 4 sets of front pads and at least 1 set of rear pads each but even at those mileages no major scoring of the discs or a serious lip on them.

biko

Unless there is an issue with the discs (warped or whatnot) then wait to change until there is a noticeable lip on the disc.

bladespin

Measure them.

sundodger5

Ideally they should be measured with a vernier calipers and compared to the minimum spec.
That said the advice above is a good workaround. Every 3rd or 4th set of pads.
If you ever let the pads go too low and you get metal to metal contact causing scoring then change the discs.
Many will say that you can get away with the damage and you can but its not the right way to do it. The right way is the safest way.

racinggreen

You would be surprised at how small the tolerance is with brake discs. 2mm under manufacturers spec would meam they need changing. Im not saying thats the same for every car but its worth checking. At tne end of the day they are a major safety component and should be changed sooner rather than later.

Bullmaccabe

I have just replaced the front and rear discs on a Mercedes E200. Not too difficult just watch out for:
1. The discs may be very tight on the hub and take some effort to remove. Rotate the disc and using a rubber mallet, strike it from the back (engine side).

2. The rear discs on the Mercedes take more effort. This E200 has disc brakes all round but also has shoes positioned inside the rear disc for the foot brake. To remove this rear disc, I had to adjust the shoes by moving them away from the disc. Then, as with the front disc, thump the back of the disc with a rubber mallet.

Finally make sure the hub is spotless before you refit the new discs. Otherwise the disc will not fit correctly and you will feel movement in the pedal when you brake.
Hope this helps.

clonagam

Great help. Many thanks