Brake disc minimum thickness

Having measured the brake disc thickness on my 996 C4S with a brake disc gauge, I think that my front discs are below the stipulated minimum thickness of 32mm (mine measured 31.6 and 31.7 for NSF and OSF, respectively). I realise that these need changing at some point but wondered how critical this is? The car needs an MOT, will this be picked up there and potentially fail the MOT?

Unless the discs are cracked or have considerable rust on them most MoT test centres will not fail you.

would you expect the brake warning light to come on when they need changing?

This only prompts for worn pads, not discs

Ah right - cheers.

However, should the discs become so thin, heavy braking may well cause a catastrophic failure which results in the the sensors being set off :lol: :lol: I don't want to be the one to test this hypothesis

Don't blame you! ;-) problem i have is don't trust OPC when they say they need replacing and too incompetent to know how to check myself! was relying on the light before changing, but guess i need to keep an eye on the discs myself ;-)

More important issues are; 1. how true/square is the disc? 2. Are the discs flat and smooth without grooves? 3. Are the discs cracked? If the above are fine I'd not change the discs. You can have the discs checked for cracks using non destructive techniques as employed on aircraft components - these include magnetising the disks (removed) and then applying iron filings - the cracks, if there are any, show up clearly. The second method is to soak the disks in a fluorescent solution and then shine a UV light on them - cracks show up as above.

Thanks for the comprehensive response, although I am not many owners would be able to answer those questions with strong conviction :? Car is due for service in a week or so and they will be looked at by a proficient Indy and they can advise me as to whether they need changing or not.

Comes from a mis-spent youth as an aircraft fuel systems engineer where I met a very famous Formula 1 word champion. BTW I wonder if that BA 777 ran out of fuel! :lol:

That's a possibility I hadn't considered... The undercarriage appeared to have punched its way right through the fuel tanks and yet it didn't catch fire! With my engineer's head on, flying against a 25mph + wind will have used a lot more than usual? Bloody hell! :shock: Another thread hijacked...

Wattie, There was a guy on one of the rolling news channels from Jane's who was being asked about the very same thing as it had not been put forward as a suggestion if that make sense. However the fire brigade did say that a large quantity of fuel leaked from the plane. Glad I am not on a 777 for a while. BACK TO THE THREAD It is a 996 so the problem with 996 discs in corrosion on the inside of the disk. When I took mine in for its first MOT the guy spotted it within a few seconds of starting his visual inspection. I knew about it and the car was booked in with 9M a few weeks later anyway. So forget how thin your discs are on the outer surfaces and check what it looks like inside them. Chatting with various mechanics I have heard horror stories many about cars where the owners did not trust the OPC as all they saw were bright shiny disks, so how could they be corroded? With the wheels off get a flashlight and look into the vent. You might be shocked by what you see. Also something else to think about with running your discs low. Do you have pads that are getting very thin too? If so you are approaching the max extension points on the pots in your brakes. Now that really is not a good thing.

Thanks for the pointers. I agree that the inside faces of the discs on a most 996's suffer from some degree of corrosion/scoring etc and mine are no exception. However, having looked at these closely recently this is not my concern, as compared to others I have seen, mine are fairing well. In most cases the general 'everyday' use that a 996 sees is not enough to keep the inner faces of the discs corrosion free. Some spirited driving every once in a while may help :wink: OPCs seem to either take the approach of suggesting pads and discs need changing every time the car is presented to them or they do not do a thorough enough check on the discs, as you suggest. For this reason, I like to inspect my pads/discs myself or use a good Indy. My pads have plenty of wear left so this is not an issue.

ABSOLUTELY I had same issue on my old 996 - looked and felt fine on outer face and completely 'shot' on inside... OPC quoted £1500 all round !! so don't use OPC - Design 911 had good prices for complete package and let indi or I used 'brake center' to change all mine. .. £650 discks & pads all round for Porsche original parts plus about £150 labour. Also remember to buy new bolts as they have a thread sealing. If you're anywhere near Nottingham use Zentrum great guys ex OPC.. http://www.zentrum.co.uk/ Good luck