checking brake discs

[thelawnet1]

Want to put new pads on my car, got an MOT advisory for 'worn, pitted or scored' brake disc on the same axle (it doesn't say near or offside though). Not sure what I should be looking at, only thing I can find is that the discs are 22mm new and replace at 20mm, though not sure how to measure these. Also replacement discs seem to be ~£20 to ~£50 depending brand not sure if it makes a difference, and 'left' and 'right', but one site sells 'left' and 'right' but only lets me buy two (surely you'd want 1x left and 1x right?)

Nearside


Offside


Also offside

[E-bmw]

My advice to you would be next time take it elsewhere, there is nothing wrong with them!

I can't comment on the pads obviously, but in discs of that condition I would be surprised if they are badly worn.

Can you see the depth of pad material left?

[thelawnet1]

E-bmw said:

My advice to you would be next time take it elsewhere, there is nothing wrong with them!

I can't comment on the pads obviously, but in discs of that condition I would be surprised if they are badly worn.

Can you see the depth of pad material left?

Well the pad doesn't look great I think? (failed <1.5mm)

[geeks]

Disc surface looks ok to me but there is quite a lip there, I would change them but I have a thing about discs with lips lol!

That pic of the caliper does look like the pads are done!

[paintman]

You measure the remaining disc thickness with a micrometer - a vernier caliper will do, but you might need to push a piece of plastic tube over the jaws to get past the lips caused by wear.

If they are at or past the specified minimum thickness then they need to be replaced as do the pads. I've not come across handed discs - yet! - although pads may have physical differences which require them to be fitted so they work on specific sides of the disc.

I've found over the years that I usually get two sets of pads to one set of discs.

Minimum pad thickness is usually also given in factory manuals but for MOT purposes it's 1.5mm:
"Reason for rejection g. a brake lining or pad insecure or less than 1.5 mm thick at any point"
https://www.mot-testing.service.gov.uk/documents/m...

Edited by paintman on Monday 27th November 19:32

[stevieturbo]

As others say, the outer surface looks to be in great condition...pads not so. And disc thickness could be questioned.

however...sometimes the outer surface can look great and the inner totally different. And with an outer pad down that far, I'd expect the inner to be much worse.

So there's only a part of the story there.

[227bhp]

The description says 'worn, pitted OR scored' and yes they are worn out, you can see a mm or so of lip on just one side.
If the internal cooling vanes are helical then they need to be put on the right side.

[thelawnet1]

227bhp said:

The description says 'worn, pitted OR scored' and yes they are worn out, you can see a mm or so of lip on just one side.

is there a loss of efficiency between 22mm (new), 21mm (part-worn) and 20mm (replace)?

[227bhp]

thelawnet1 said:

227bhp said:

The description says 'worn, pitted OR scored' and yes they are worn out, you can see a mm or so of lip on just one side.

is there a loss of efficiency between 22mm (new), 21mm (part-worn) and 20mm (replace)?

The pedal shouldn't feel longer, but it does, after fitting new discs and pads it'll feel a bit better, but that isn't efficiency so no is the answer. It isn't about that though, as they get thinner they get weaker so more prone to breakage and overheat if really abused. They aren't dangerous as they are, they're just heading that way and it's fair to say that the manufacturers recommendations will be well within limits.

Don't fall into the trap of thinking an MOT is something to try and get around or a tiresome expense, the attitude should be 'Thanks, you've just highlighted something that will be dangerous given time'.

[thelawnet1]

227bhp said:

Don't fall into the trap of thinking an MOT is something to try and get around or a tiresome expense, the attitude should be 'Thanks, you've just highlighted something that will be dangerous given time'.

Well there's always a balance. On the one hand you might as well do them both as labour is significant part of the cost, on the other I only do 4500 miles a year so it's not really clear they will ever become dangerous as long as I own the vehicle, and then on the other it's only another £60 or whatever (depending on brand).

And so on.
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[227bhp]

thelawnet1 said:

227bhp said:

Don't fall into the trap of thinking an MOT is something to try and get around or a tiresome expense, the attitude should be 'Thanks, you've just highlighted something that will be dangerous given time'.

Well there's always a balance. On the one hand you might as well do them both as labour is significant part of the cost, on the other I only do 4500 miles a year so it's not really clear they will ever become dangerous as long as I own the vehicle, and then on the other it's only another £60 or whatever (depending on brand).

And so on.

Yes I agree, that's why it's an advisory, rather than a fail. The other factor is; do you spend money putting new pads on old discs?
I'd say the MOT station have been very fair, basically you've now got all the facts (including those provided by yourself) and can make an informed decision based on those.

[geeks]

If its all apart to do the pads then I would do the discs as well looking again at the pics I wouldn't be happy with that lip myself especially on the fronts, especially for the sake of £60!

[thelawnet1]

I actually ordered some discs, but they came too late and turned out I had some pads kicking around that were supposed to have been fitted previously.

have now been fitted by the axle on which they were fitted is making a sort of squealing noise, just driving along 1 mile or so from garage. Not sure if that is normal and they should settle down or?

[SebringMan]

thelawnet1 said:

I actually ordered some discs, but they came too late and turned out I had some pads kicking around that were supposed to have been fitted previously.

have now been fitted by the axle on which they were fitted is making a sort of squealing noise, just driving along 1 mile or so from garage. Not sure if that is normal and they should settle down or?

It may be the lip catching the old discs. Or the pads are seized into the carrier ; did you have to force the pads in with more than hand pressure and were they free to move on the carrier?

[thelawnet1]

SebringMan said:

It may be the lip catching the old discs. Or the pads are seized into the carrier ; did you have to force the pads in with more than hand pressure and were they free to move on the carrier?

I dunno I didn't do the pads. However all seems ok now, so it must have just been a newly fitted thing.