Front Discs - Stealers Opinion

Their way of getting this fixed sounds about correct. Normally pistons seize due to a torn dust cover and the onset of rust on the piston. This happens to the part of the piston that extends out of the cylinder. Once installing new pads you´d have to push the piston farther inside which will not work with a rust collar.

If not corroded badly one can sand the piston down a wee bit and replace the dust covers for the a.m. roughly 25 quid. Labour will be likely 1hr+. There is no guarantee though that the system remains sealed.

If piston and cylinder are corroded badly you will indeed need to get new/used working calipers.

WD40 and a hammer maybe cheap but will not help here

Cheers
Reinhard

The repair kit is just an o-ring and a dust seal.

The only problem is that you cannot buy new pistons from BMW.

HOWEVER.
Bimmerworld in the US have stainless pistons for 78usd/pair.
I just bought a set for my e46 M3, which happens to have the same piston as the 850i, and the e36 M3.

http://store.bimmerworld.com/s…caliper-pistons-p755.aspx

Note that they do not accept web orders before you have done 4 orders, so it is a bit ineffective to have to contact them, get and fill in an order form, then wire money via your bank.
But the pistons look really good......

I did glass blast and powdercoated my 850's calippers last year and they are like new.

This year my M3's ones gets the same treatment, AND new front pistons as I've had one piston seize on me this winter due to a bad dust seal and is badly corroded, temp fixed with steel brush.

EDIT:
If you have the OEM single piston calipper (CSi have 4 pistons), not sure what the 840 has. I suggest you get a repair kit, the rubber sliding guides new front pistons and the bleed nipples.
If you are able to, get them blasted and just get caliper paint on a rattlecan and paint them, they will look better, and hold up better too.

EDIT2:
Based on reinhard's post you might check out eBay for brembo 4 pot calipper pistons.

FYI: Brembo 4 pot brakes on 840 M62´s.

Cheers
Reinhard

Zitat von reinhard;87618

FYI: Brembo 4 pot brakes on 840 M62´s.

Cheers
Reinhard

That was something I was not sure of so I edited my post

Try this place:

http://www.brakeparts.co.uk/

They are the only place I found that sells individual parts for the Brembo brakes fitted to the 840. One of my calipers developed a leak from inside one of the cylinders, probably due to corrosion of the piston damaging the seal when it was pushed back in. I bought replacement seals from the place above, stripped the caliper and cleaned out the bores. The key is getting it immaculately clean before re-assembly.

Once the caliper was back together with new seals, pistons and dust caps, it worked like new. I could push the pistons all the way back with just finger pressure, no tools required. Try that with just about any Brembo 4-pots and chances are at least one piston is at least party seized.

For reference, here's a shot from the webpage (can't do a direct link because it's Flash )

The inside pistons are the ones that regularly seize on these calipers. The problem is getting them apart once seized and if bad, they can score the inside of the cylinder and thats terminal. The pistons on these are alloy so its normally that white crusty corrosion and not rust that causes the problem.

The way the outer seals are retained on these calipers is pants and as soon as the seal comes away from the caliper, seizure is just a matter of time.

If you can get them apart and the bores are not scored or the pistons pitted then a set of seals should do it. It shows calipers available on that link but they are not.

8Tech.

I usually just remove the pads and pump the braked until the pistons pop out.
However on multi caliper pistons this may be an issue as once the fist pop out pressure is gone and the others will not come out.
Should be possible to work around this though.

Zitat von 8Tech;87663

The inside pistons are the ones that regularly seize on these calipers. The problem is getting them apart once seized and if bad, they can score the inside of the cylinder and thats terminal. The pistons on these are alloy so its normally that white crusty corrosion and not rust that causes the problem.

The way the outer seals are retained on these calipers is pants and as soon as the seal comes away from the caliper, seizure is just a matter of time.

If you can get them apart and the bores are not scored or the pistons pitted then a set of seals should do it. It shows calipers available on that link but they are not.

8Tech.

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As usual, spot on. When I rebuilt mine with new pistons I had to spend ages removing corrosion, particularly in the grooves which seat the dust boots. When I reassembled I deliberately refitted the boots while the high temp paint was tacky in the hope that will help the boots stay put. Ropey design.