Brake discs replace with pads?

smr · 14 Mar 2025 at 09:26

I need new rear pads and I am wondering if I should replace the discs at the same time.

The discs were 10.65mm as of January 7th this year and I believe they start at 11mm.

Car is Focus MK4 ST Line X.

Is it best practice to replace the discs whilst changing the pads or should I leave the discs?

Armageus · 14 Mar 2025 at 09:30

The majority of cars I've had, it's normally a case of replacing the Discs with every 2nd set of pads.

JonRGV250 · 14 Mar 2025 at 09:33

If they’re not below min thickness and not excessively corroded, pitted etc, then just replace the pads.

smr · 14 Mar 2025 at 09:37

JonRGV250 said:
If they’re not below min thickness and not excessively corroded, pitted etc, then just replace the pads.

9mm is minimum. I can have a look for corrosion etc.

smr · 14 Mar 2025 at 09:38

Armageus said:
The majority of cars I've had, it's normally a case of replacing the Discs with every 2nd set of pads.

I've had my car for a year so not sure on that.

smr · 14 Mar 2025 at 10:20

Here's a pic

smr · 14 Mar 2025 at 10:43

Does it look like they should be replaced ? Look quite rusty.

JonRGV250 · 14 Mar 2025 at 11:25

The corrosion won’t make a single jot of difference to the braking performance.
At worst you may get a slight grinding where the pad touches the edge of the rust, but that’s it.
That said, they’re solid discs, so won’t be too expensive to replace.

Nasher · 14 Mar 2025 at 12:23

The edges are crusty but it's fine. They may start to warp though if rusting un-evenly in places you can't see.

alexp999 · 14 Mar 2025 at 12:24

smr said:
Here's a pic

Mine look like this on my MK4 ST and I was advised to get both pads and discs done on rear at my next service. (The rear discs on the ST are the same as the rest of the Focus line-up, it's only the fronts that are different).

I don't know whether to get Ford OEM again, or stick something on that's coated so they don't rust like this next time.

JonRGV250 · 14 Mar 2025 at 12:36

If the pad isn’t contacting the edge of the disc it’ll corrode, not much you can do about it.

alexp999 · 14 Mar 2025 at 12:42

JonRGV250 said:
If the pad isn’t contacting the edge of the disc it’ll corrode, not much you can do about it.

Isn't the idea of the coated discs that the only exposed parts that could in theory rust are just the contact area?

smr · 14 Mar 2025 at 13:20

alexp999 said:
Mine look like this on my MK4 ST and I was advised to get both pads and discs done on rear at my next service. (The rear discs on the ST are the same as the rest of the Focus line-up, it's only the fronts that are different).

I don't know whether to get Ford OEM again, or stick something on that's coated so they don't rust like this next time

Do you know which size discs they are, on ECP it throws up two sizes - 271mm and 302mm.

So I'm not sure which to buy.

dave28 · 14 Mar 2025 at 13:48

smr said:
Do you know which size discs they are, on ECP it throws up two sizes - 271mm and 302mm.

So I'm not sure which to buy.

if ever unsure, car dealerships have a parts office who you can phone and ask. They ask for your Reg plate and look it up on their computer system.

alexp999 · 14 Mar 2025 at 15:07

smr said:
Do you know which size discs they are, on ECP it throws up two sizes - 271mm and 302mm.

So I'm not sure which to buy.

Looks like it depends on the engine and also if it is Estate or Hatch.

I usually use EBC to look up parts, you can then search their part number to see exact size:

https://www.ebcbrakes.com/vehicle/uk-row/FORD/Focus/

For the EBC Standard Rear Discs, D2184 is 271mm and D2031 is 302mm.

Diddums x · 14 Mar 2025 at 20:22

Nasher said:
The edges are crusty but it's fine. They may start to warp though if rusting un-evenly in places you can't see.

Listen to this guy, he knows loads about cars.

BUDFORCE · 14 Mar 2025 at 20:25

smr said:
Do you know which size discs they are, on ECP it throws up two sizes - 271mm and 302mm.

So I'm not sure which to buy.

If you take the wheel off the disc size is usually stamped onto the caliper somewhere.

Or failing that use a tape measure.

Jupiter · 15 Mar 2025 at 13:05

No need to replace those discs.

smr · 15 Mar 2025 at 13:48

Will these discs damage the new pads though?

I don't particularly like hearing grinding noises if it's going to cause that.

Also, as I am getting new pads fitted next week, would it just make sense for the sake of £40 to ask them to put new discs on as well? I'm guessing it doesn't take much more time to put new discs on or does it ?

andy_mk3 · 15 Mar 2025 at 13:58

I'd replace them personally. While they are fine for now, rust creep is starting on the outer edge, and will continue to get worse over time. The braking surface doesn't look amazing either. Perfectly ok to not replace them, but personally I would do it while it's being done.