How corroded to discs have to be before they’ll fail an MOT?

Dunno – but I’ve had it happen too many times. Use the car more during the winter helps.

Not sure but my car just passed with an advisory as the front discs were below minimum thickness. So they need changed but not badly enough to fail an MOT apparently?

Not sure there’s a hard and fast rule – depends if the tester’s in a good mood or not 🙁
My car got an advisory on the rear disks being corroded last year so I’m going to change them myself before the next MOT.

They became corroded when we were away for 6 months.

Not sure but my car just passed with an advisory as the front discs were below minimum thickness. So they need changed but not badly enough to fail an MOT apparently?

Yet they failed mine for a broken/corroded Reg plate light!

Don’t normally have time to get the cars in for MoT myself, so local garage gives them a look over and takes them. Rear discs on the MX5 are prone to rust as the pads just make a small contact are on the discs, they just use a drill and wire brush to see them through.

Yeah my Van failed the MOT on an “imbalance” on the front brakes this time around. The Van had been standing for 4mths whilst not in use and the front had the normal corrosion/rust buildup from not being used. So memate got an angle grinder and a wire brush pad on it and simply went around the disk front and back. Passed it’s MOT, no new discs required, no advisory.
I’d recommend you do this, it took about 2 hours to do both.

they just use a drill and wire brush to see them through

I might try that first.

Just pulled the back wheels off my car and attacked the disks with a bronze brush on an angle grinder. Did a nice job of removing what turned out to be some fairly thick rust and the disks look a lot better. A quick test drive reveals that some pulsing, that I thought might be thick areas of rust hitting the pads (yes they were bad), now seems to have gone.
Main problem is that this car isn’t used so much now we’ve got a more economic one and as such stands around allowing rust to build up.

New discs aren’t that expensive anyway and are easy to fit with the right tools.

You will feel a bit of pulsing, just until the pads resettle on the discs.. My Van did and it’s fine now, can’t feel a thing, except braking of course.

Well done you anyway.

It can be done on the thickness of the disc itself, or the braking effectiveness ie either an inbalance between the left and right side, or not enough braking force through both, a bit of corrosion shouldnt matter so long as the discs are thick enough and can stop the vehicle effectively.

Should have driven around more with the handbrake on 🙂

I did that, the rear shoes had stuck solid.. I shudv’e left the Van in gear rather than on the handbrake. It came off eventually, once I’d reversed over the neighbours garden….

A small hijack my car just passed MOT but got an advisory about pad thickness should I be changing discs and pads or just pads and how to tell if rotor knackered?

Thanks..

TBH the price of most discs it’s not even worth 2 hours with an angry grinder, but some cars are pricey. For them a cup brush on the grinder will do the trick nicely.

However you’ll be surprised what will pass easily, so long as there are not deep scores.

Don’t be tempted to use the pads to scrape it off unless it’s just a surface dusting – it embeds rust in the pads which then rip your rotors to pieces over the next few months.

A small hijack my car just passed MOT but got an advisory about pad thickness should I be changing discs and pads or just pads and how to tell if rotor knackered?

Pad thickness fails at 1.5mm, so I guess you’re around 2mm. Has no bearing on disc thickness, but I’d change them if they’re deeply scored or have large areas of corrosion not braking properly, otherwise no issue. Simple answer is measure the disc thickness taking care to remember there’s a lip on the outside of the rotor due to wear normally.

New discs aren’t that expensive anyway and are easy to fit with the right tools.

Depends on the car.

Some put the disk inboard of the hub making it a PITA, having to dismantle the hub as well.

Some integrate it with the hub, meaing new disk = new hub = expensive (but easy to build in the factory).

Some people would rathe rjust hand it over to a garage to do. I’m coming round to this idea after yet another brake related fault on the midget!

Ohhhhh don’t talk to me about 1275cc Midgets..

TBH the price of most discs it’s not even worth 2 hours with an angry grinder

5 mins to remove wheel and 5 mins with the grinder. 30 mins tops including a coffee.

5 mins to remove wheel and 5 mins with the grinder. 30 mins tops including a coffee.

4 Wheels, and need to do both sides of the discs not just the front. Not quite 30 mins, even from your own timings 😉

Fronts not rusted (would be in trouble if they were!) so only two rear disks to do – and can only do one side as the inside is completely covered by a backing plate.

Corrosion on disc surface from lack of use isnt a problem. Corrosion when car is in use may be a sign of poor caliper/pad performance. Corrosion round the edges where pads dont contact discs can build up and grind against the pad backplate resulting in brake drag, easily fixed with angle grinder. Amazing how freely car rolls along. Saves fuel too!

If its casuing brake imbalance on MOT rollers then replace discs and pads. Discs are soooo cheap these days.