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Skimming Brake Discs

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Skimming Brake Discs

Postby Jeffs530d » Wed, 21 Oct 2009 15:14:58 UTC

Hi all,

Has anyone done this? Is so what is the minimum thickness at which BMW would change the discs altogether and where it is measured?

Reason is the off side calliper is groaning. I have seen the posts recommending cleaning the pistons etc, and whilst the pads are not too worn I was going to change them anyway whilst I had the calliper off. Just for good measure and provided they were still within the wear spec my Father in Law runs an engineering firm (makes bits for F1 card so knows his stuff) and has said he can can skim them for me like they did in the good old days.

Is this acceptable now-a-days or do I need my head examined? :shock:

The car is a manual and really I am very light on the brakes so I tend to make consumables like this last ages. I dont want to replace expensive discs unless they are close to or over the limit. That why BMW set a limit!

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Re: Skimming Brake Discs

Postby slim_boy_fat » Wed, 21 Oct 2009 15:45:14 UTC

Jeffs530d wrote: ...my Father in Law runs an engineering firm (makes bits for F1 cars so knows his stuff)


Get him to make some carbon fibre ones for you..... :lol: :lol:

Skimming by someone with the necessary facilities/expertise [which it sounds like he has in spades 8-) ] will do no harm as long as the discs remain within tolerances, but tbh unless they're badly scored or your car has been to the moon and back I'd doubt it being needed.

Replacement discs aren't too expensive in the grand scheme of things in my experience.
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Postby roadhog » Wed, 21 Oct 2009 16:46:05 UTC

Skimming discs seems to have gone out of fashion with the introduction of vented ones. If you're just talking of taking the lips off the outside it shouldn't be a problem but heavily scored faces will easily take you beyond the limit. The minimum thickness is usually stamped on the outside edge of the disc but as above, new discs aren't that expensive.
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Postby terminator » Wed, 21 Oct 2009 16:51:34 UTC

As Slim Boy Fat said, new discs are relatively inexpensive these days, particularly if you get the OEM make (Jurid, Zimmerman or ATE) from motor factors such as ECP or GSF.

I always change my discs when I change the pads, may sound a bit over the top, but reassuring none the less.
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Postby BorrySG » Wed, 21 Oct 2009 19:27:26 UTC

Nothing wrong in skimming discs provided they are kept within specified tollerances and being vented one sde has not suffered disproportionately more wear than the other side of the central vent. Your Father - in - law will know how to check and have suitable micrometer etc.
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Postby Ranger 6 » Wed, 21 Oct 2009 19:29:36 UTC

Look on the disc edge for the wear indicator, as has been mentioned it's pressed into the outside edge.

Image

Obviously when the raised portion is worn then the disc should be junked. If the skimming is likely to take the disc close to it then the £70(ish - I'm guessing) for a pair of new discs would be well spent.
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Postby AlwynMike » Wed, 21 Oct 2009 20:11:49 UTC

[font=Comic Sans MS]Ranger4,
Are you sure that the picture shows wear indicators???
I thought that it was just machined to allow the disc to be clamped during manufacture.

Check your disc thickness carefully. My car needed discs at its first pad change (40-odd thousand miles). I too am pretty light on brakes most of the time - it's a manual car too, so it has engine braking! I mic'd mine up and they were just on the limit, so although they weren't scored and looked very meaty, they got thrown away.

I like good brakes, so I decided to go for EBC slotted discs and Redstuff pads - still cheaper than getting BMW discs from the $tealer. The slotted ones are supposed to be quieter than the drilled, but I do notice some noise, particularly under light braking. They do work very well when you push them hard though!

So, if you have the thickness, skim if you want to, but as has been said, discs aren't that expensive.

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Postby Peter » Wed, 21 Oct 2009 21:26:14 UTC

Ranger 6 wrote:Look on the disc edge for the wear indicator, as has been mentioned it's pressed into the outside edge.

Image

Obviously when the raised portion is worn then the disc should be junked. If the skimming is likely to take the disc close to it then the £70(ish - I'm guessing) for a pair of new discs would be well spent.


No, no, no, that is the 'balance' machining. You need to know the minimum thickness tolerance for any skimming.

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Postby BorrySG » Wed, 21 Oct 2009 22:32:59 UTC

Yes, Peter is correct, these are just balance cut- aways. If the disc was skimmed back to these is would be unuseable. I can't remember without checking but I would be surprised if you could acceptably take much more than 1.5 mm off each side of the disc if that. Min thickness should be stamped on disc. (Thickness on my 530i is given as 28.4mm in the Haynes manual for the larger 324mm diameter front discss fitted to this model).
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Postby BorrySG » Wed, 21 Oct 2009 22:49:58 UTC

As a matter of interest, I had my discs skimmed on the car. The calipers are swung away and a special lathe is attached which turns the hub and trues everything up, compensating for any run out of the hub to boot. It cost me £80 but I have special grooved and dimpled discs that cost about £120 to replace and it saved me the effort of taking them off and replacing. It would probably have been cheaper to fit new standard discs though, one of the reasons why not too many are skimmed nowadays and of course some are found to be too worn to skim.
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Postby Ranger 6 » Wed, 21 Oct 2009 23:17:30 UTC

Peter wrote:...that is the 'balance' machining.....


Interesting, thanks. I'll go and :banned: the PH massive for thier complete lack of knowledge when I posted the same pic six months ago.... :oops:
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Postby Jeffs530d » Wed, 21 Oct 2009 23:47:34 UTC

Thanks for all the superb feedback folks. :D

I see £70 was mentioned for a pair. I didn't expect them to be this cheap as I was quoted £105 just for the pads, by the time I have got the old man a bottle of scotch for skimming them I may as well have got new ones. I am off to look on ebay etc.

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Postby m330d » Thu, 22 Oct 2009 07:41:19 UTC

The minimum thickness is probally around 90% of the original ,it should be marked on somewhere, unless you are getting mates rates it will be more cost effective to get new ones.
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Postby scatmancraig » Thu, 22 Oct 2009 13:13:29 UTC

When i last did the front discs on my 530d, i got a pair of Brembo discs from GSF. I didn't ask for Brembo ones, that was what they had when i asked for front discs!!

Anyway, my discs have a shallow dot on the surface of each disc, which i assume is some kind of wear indicator. It's a similar thing that you seen on slick tyres, where if you cannot see the dot anymore, then the tyre or disc will need replacing. As a slick tyre doesn't have any tread, you need the wear indicator to show you when to change the tyre! Same with a disc surface, as there is no other obvious was without properly measuring the disc.
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Postby AlwynMike » Thu, 22 Oct 2009 14:43:15 UTC

[font=Comic Sans MS]My (standard) discs had no wear markings, or any stampings to tell me the minimum thickness.
I don't think the EBC had any markings on them either, apart from the directional arrow (yes, they're handed for each side of the car - pity I didn't notice it until I'd already fitted one side :oops: )

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Postby szjon » Thu, 22 Oct 2009 15:00:15 UTC

Had mine skimmed after overheating them on an overloaded drive to Poland, worked a treat 2 years ago and are still working perfectly. They're cheap enough but if you have a decent tolerance left I'd go with the bottle of scotch, keep it in the family! :-)
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