MOT brake disc pitting...what's the limit?

[macthejaffa]

Howdo all......Had a recent MOT fail on various bits which I've since put right myself, but one thing has got me a bit confused...According to the fail sheet the offside disc is excessively worn and distorted...Fair enough, I thought, I'll just get have to get some new ones come payday, but I got the wheel off and decided to check it out for myself, and to be honest it looks in tip top condition apart from one very small patch of pitting on one side of the disc. I couldn't quite believe this was enough to fail the MOT on, so have decided to throw it out amongst all you brainy lot for some expert opinions....I'm going to try to attach a pic of the very small patch in question, and I hope someone can shed some light on what is the limit for this pitting on the discs, or is it automatic fail at the slightest sign of any? I'm confused and any help much appreciated...Cheers all

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[Oldiebut goodie]

Get a micrometer across the disk and compare it to the thickness of a new one. What is the run out like? If you can see a wobble when you turn it that is what they are on about - not pitting. You say worn and distorted is on the fail sheet but you have only looked at the surface condition.

[CovKid]

In the end its costlier to argue it. For the price they are, just fit some new ones. You'll get better braking anyway (at least to start with )

[Aidan]

those are close to the wear limit anyway judging by the raised lip there's only 3mm wear built in to them

[waltraud]

Is there a way to measure the thickness without a micrometer thing?

[Oldiebut goodie]

I would take Aidan's word for it - his eyes are most probably better than mine on that small pic. Plus the tester has had more experience.
Bite the bullet and replace them, it makes sense.

[Aidan]

I wouldn't take my word for it I'd check for myself
I KNOW the syncro discs are 16mm and the wear limit is 13mm, I'm not certain of the figures for the 2wd,
regards measurement you can pick up a cheap pressed steel vernier in the tool section of some £ shops, accurate enough for this job, though they tend to be a fiver plus else where
the wear limit is stamped on the edge on genuine discs, maybe on some aftermarket ones too