MOT Advisory-How long should I leave it

Joe_Horner

Lum wrote: »

Don't a lot of manufacturers specify a minimum thickness that the brake discs can safely wear to?

Yes, often cast into the hub of the disk for convenience when re-facing.

Not that anyone bothers refacing them now, but the very fact that the manufacturers say you can put them in a lathe and skim maybe a mm or two off each side to remove scoring (GM don't recommend refacing for scores less than 1.5mm deep) is proof that fairly considerable wear (that would certainly be advised for MOT) is safe!

molerat

I usually take my discs off every year, or two depending on how they look, chip off the edge crust with a cross pein or welders hammer and give the faces a going over with either a wire cup brush or coarse sanding disc in a drill or angle grinder. Many discs are condemned as "worn" when it is just a build up of crust on the edge.

red_eye

WLC wrote: »

Hi all

Got an advisory for both "nearside and offside front brake discs worn, pitted or scored, but not seriously weakened".

Only ever failed or passed with this before. But with it being an advisory, I just wondered when I should look at changing them? I'd obviously have my pads done at the same time.

Am I right in thinking I should just get them done asap, as obviously they will just get worse, not better. Or are they normally still good for X amount of miles

Thanks all

change as soon as possible, just becuase it is an advisory it does not mean it is ok to drive till next pay day or xmas.

dont want to scare you, but all it takes is for one little crack to develop (on a vented disc the braking serface is already thinner compared to a solid disc) which will get bigger and bigger as the disc cycles from hot to cold and then when you least expect it you press the brake and BANG... the disc is in a million bits

red_eye

Joe_Horner wrote: »

Because some places rely on that thinking to sell you brakes that you don't need. It is NOT dangeroous at the moment (or it would have failed), so getting a second opinion is perfectly sensible.

I don't think anyone on here is sayting just carry on and forget it, but advising for worn disks is one of the favourite ways to drum up business. It used to be failing them for wear, but VOSA finally put a stop to that, so now they simply advise you and rely on the "Oh NOES, it's SAFETY!!!" fear to sell them anyway.

If you don't know enough to judge them yourself then you can either accept their "advice" or ask the opinion of someone else who does know what they're looking at and see if they agree.

As an example, here is a disk with wear (notice the lip at the outer edge) which an MOT tester would be perfectly entitled to advise for because it is worn. But it's nowhere near "worn out" and you'd be a fool to replace it at this stage:

that is a solid disc it is nowere near the min thickness but in this day and age nearly all cars have vented discs and with that much lip the vented disc will be near or at the min thickness.

to add to my last post on vented discs there is a cut out

red_eye

molerat wrote: »

I usually take my discs off every year, or two depending on how they look, chip off the edge crust with a cross pein or welders hammer and give the faces a going over with either a wire cup brush or coarse sanding disc in a drill or angle grinder. Many discs are condemned as "worn" when it is just a build up of crust on the edge.

by faces I hope you do not mean the braking surface.
Doing this would mean that your pads will have to bed in and as you have also gone over the braking surface with an angle grinder the pads will be constantly bedding in with all the irregularity, as there is no way for a human to free hand a perfect flat and true surface

molerat

Yes, all sanding them does is removes a wee bit of the shine, it does not remove any metal.

Lum

red_eye wrote: »

by faces I hope you do not mean the braking surface.
Doing this would mean that your pads will have to bed in and as you have also gone over the braking surface with an angle grinder the pads will be constantly bedding in with all the irregularity, as there is no way for a human to free hand a perfect flat and true surface

Personally I wouldn't go beyond taking the lip off, which I have had done, not sure I'd fancy letting someone take any sanding or grinding tools to the face itself.

I can see why it's necessary though. Most "warping" is actually just an uneven build up of pad material because some plonker got their brake discs nice and hot and then sat at a set of traffic lights using the footbrake instead of the handbrake, causing a fraction of a mm of pad material transfer to a single spot. It's more common on automatic cars for obvious reasons.

So next time the car in front with it's eye-level nuclear furnace brake lights is burning away at your retinas at a set of lights, just remember how much money they're wasting.

Ratboy

Lots of quotes against me, regarding replacing safety related equipment, so that confirms that humans are idiots. You are told your vehicle is potentially dangerous, but many people say 'Don't bother, if a testing station told you they were on the way out, and failed government standards, I'll use them anyways, even though, if they break, I'll die', and possibly take somebody else out too..That's the attitude these days, look after yourself, and ignore all others especially in driving.

My car was such an issue for my neighbour, where it was parked, so he set fire to it. Nice! And the Police do. NOTHING.

Rolandtheroadie

Ratboy wrote: »

Lots of quotes against me, regarding replacing safety related equipment, so that confirms that humans are idiots. You are told your vehicle is potentially dangerous, but many people say 'Don't bother, if a testing station told you they were on the way out, and failed government standards, I'll use them anyways, even though, if they break, I'll die', and possibly take somebody else out too..That's the attitude these days, look after yourself, and ignore all others especially in driving.

My car was such an issue for my neighbour, where it was parked, so he set fire to it. Nice! And the Police do. NOTHING.

The brake discs have been reported as "worn, pitted or scored, but not seriously weakened".


What government standard have they failed?

Lum

^^^ That one is the worst fault description. "Pitted" basically means "they have a bit of rust on them", but brake discs are lumps of bare metal, just leaving your car sat on the driveway for a wet weekend will cause them to rust to the point where you can get an advisory for being "pitted". If you then drive straight to a local MOT station on Monday morning without cleaning them up, you'll likely get an advisory for that. The fix is to do a few moderately hard brakes from 70->20 to scrape all the rust off.

Ratboy wrote: »

Lots of quotes against me, regarding replacing safety related equipment, so that confirms that humans are idiots. You are told your vehicle is potentially dangerous, but many people say 'Don't bother, if a testing station told you they were on the way out, and failed government standards, I'll use them anyways, even though, if they break, I'll die', and possibly take somebody else out too..That's the attitude these days, look after yourself, and ignore all others especially in driving.

Your attitude only works if you assume that your MOT tester is a paragon of virtue who has absolutely no interest in making further money from you for doing unnecessary work. Unfortunately the real world doesn't work like this.

They can get away with it easily enough because the MOT is a visual inspection they don't measure your brake discs with a micrometer and they become "worn" the first time you use your brakes. The tester can say "that was my opinion" and nothing can disprove this.

We don't know where the MOT test was carried out, so lets do a little thought experiment here and pretend that it was Kwik-Fit. Would you still be saying "Oh it must be about to fail" and not "Oh the spotty little toe-rag is trying to scare me into spending unnecessary money".

Also please explain why my discs that got an advisory three years ago did not get an advisory last year or the year before? Did the discs magically grow new metal and fill in any grooves or cracks? or perhaps was the MOT tester from three years ago just a slimly little git who assumed I was a woman who knew nothing about cars and try to get an extra few hundred quid out of me for new discs that I didn't need.