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MOT failure. AAaarrgghh!

Maxy M

24 May 2004
Rear faces of discs corroded (pitting) - all corners. Car now booked into BS Motorsport to have them replaced.

When Peter Morgan PPI'd the car 9 months ago all was OK - and took photos to prove it. So can this be caused just by normal use? Have I been carelessly putting the car away wet? (I try not to). Or... what? Any thoughts welcome...

Migration info. Legacy thread was 31132

I think that disc corrosion is very subjective at MOT's. I replaced my rear discs as they were heavily corroded on the inner fces at my last service but that was immediately after it had passed an MOT.

The discs didn't actually require replacement because they were too "thin", I just decided to change them because the pads needed changing as well.

The problem is that 993 discs have a high friction coefficient as they are made with using carbon steel and ths is more prone to corrosion. As you say, try not put the car away with wet discs, how you do that is another matter....


Migration info. Legacy thread was 31136
can't you just get them skimmed?

Migration info. Legacy thread was 31140
Hmm... Skimming could be an option; thanks Noony.

How come the outer faces are lovely and shiny, but the inners are pitted? (You can tell I'm a technical doughhead.)

(Quite honestly, when I looked at them I couldn't see anything that would seriously risk endangering me or any other road user. I did wonder if I had an over-zealous examiner...)

Thoughts still welcome.

Cheers, mm

Migration info. Legacy thread was 31149
If the inside face is pitted surely this means that your pistons are not working on the inner face?

Migration info. Legacy thread was 31151

I have had a similar problem. I believe the inside corrodes more easily because of the proximity of the heat shield. Have had the fronts skimmed once which made a big difference. We had an open day at OPC colchester yesterday. They did and inspection and they suggested that at the next service the rears should be skimmed and they will do this next week. I believe some independents have the Porsche approved machines that will do the job with the discs (rotors) on the car. Only limitation is if the final thickness of the disc would fall below the minimum tickness, then skimming is not an option. Off hand I do not know what this limit is. My OPC also cleaned up the pads to ensure that they were flat to the disc.

For racing and lots of track days perhaps this is not the solution but for normal road use I believe it is perfectly acceptable


Migration info. Legacy thread was 31154
common guys you all should know this has affected 911's for years, the inner faces corrode, never wash a car and not drive it round the block to dry the discs,

I was told the brakes are made by Brembo and its because of the steel they use?

you can skim them but Porsche dealers check the thickness of the disc when they are serviced

Nature of the beast i'm afraid

Migration info. Legacy thread was 31159
Thing is, I always do drive the car round the block after washing it. Maybe my block needs to be bigger...

Migration info. Legacy thread was 31162
Just a follow up.

Had my rear discs skimmed last week by OPC Colchester, in conjunction with annual service and inspection.

Big improvement in the feel of the brake pedal and the slight pulling that was there before has now gone. Cost £86.

I have now had all discs done and think it is well worthwhile. And yes when I have washed the car I always take it for a run to dry everything off.

By the way when I was at OPC Colchester had a look at the Ferrari showroom very interesting. Many cars on display outside. All outside cars, Ferraris and Bentleysl had discs rusted to some degree.

I think that rust and time is where the real problem comes. Light rust that will rub off quickly OK. If left for a long time then it starts to pit. Then problems. However my 993 shows signs of rust much more quickly than my BMW


Migration info. Legacy thread was 31581
I went for the quickest route and had new discs (bye 500 quid...). But the calipers were a bit lazy and BS Motorsport (seem good guys) near Aylesbury cleaned them up to improve their action. But they said that owing to their age the brakes do need a thorough work-over to get them back up scratch. Doh! More expense. But I get it's worth it...

Migration info. Legacy thread was 31583
My understanding of the situation regarding brake disc corrosion is this.

Up until say 7 or 8 years ago, discs were manufactured with a chromium content. Due to some EU legislation, the material composition has been changed, where by there is an increased iron content (at the expense of the chromium).

This explains the rapid oxidisation following washing.

The corrosion will be rapid if it the moisture is not evaporated or dried off, immediately after washing.

don't forget the "run around the block" - it could be very expensive.

Migration info. Legacy thread was 31593
Title should read MOT "deferred success" :D

Migration info. Legacy thread was 31594
Previous poster said:
Quote: Originally posted by gregsy on 21 July 2005

Title should read MOT "deferred success" :D
Ah, yes! I wish I'd known of this new replacement term at the time. I'd have felt so much better about the whole thing... :wink:

Migration info. Legacy thread was 31607
more reason for you to try a track day to give your discs a good work out !

Migration info. Legacy thread was 31618
Well yes, Sundeep. But I couldn't face being overtaken at great speed by all you seasoned trackdayers - or, worse, ending up coming off big-time... :eek: :oops:

Migration info. Legacy thread was 31619

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