5 series front disks

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Bmw 5 series car went into the garage a few days ago for new brake pads.
TO my horror the dealer wanted to change the disks. The car has only done
20000 miles.
Can anyone tell me what the diamter of the front disks should be on a 525 msport 2005 ???
Many TX
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On Tue, 19 Jun 2007 16:16:19 +0100, <φΏτ ©> wrote:

Eh! Discs wear thin not smaller. OD is about 335mm depending on model.
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<φΏτ ©> wrote:

If you want to check them for wear, it's the thickness that matters. The minimum is stamped on the disc hub. If this is the first change it sounds like you are not hard on brakes. But anyway, most last for two sets of pads. But estimating how long the discs may last is guesswork, and garages tend to load the cards on their side if you're paying.
If you don't have a micrometer measuring them can be tricky due to the lip on the outside. One way is to use twist drills of known size (say 3mm) as spacers and then used a vernier caliper which can be bought quite cheaply. Subtract the sum of the drill sizes from the reading.
It's one of these jobs you really need to do yourself (it's pretty easy) or use an independant. In the UK I saved about 400 gbp over dealer price when changing them all myself using OEM parts and it was an easy morning's work including collecting them.
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Dave Plowman (News) wrote:

You can get a cheap-o, Chinese, inside micrometer for about $20 which will certainly be good enough for this use and all but the most precise of measurements.
--
-Fred W

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What Fred said.
Don't mess around with drills as spacers, just buy the right tool for the job.
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Have you actually tried my method? I merely offered it as an alternative to a micrometer as a digital readout caliper is a much more versatile tool and easier to read. It's not of course as accurate but good enough for this job.
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wrote:

No. I got the right tool.
I also have the ability to read a micrometer without taking it off of the material I am measuing.
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You had to buy a micrometer for this task?

Just as well with a disc as you won't be able to remove it. And many people wouldn't have a clue how to read a micrometer correctly.
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wrote:

Sorry you are confused, I should have said, "I've got the right tool."

Exactly. Then, add the complexity of of measuring a drill bit and subtracting the value, and you have a recipe for measuring wrong and doing the wrong math and ending up with an unserviceable rotor on a car.
Buy the right tool and measure properly.
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You don't need to measure a drill bit - it's stamped on the shank. In the same way as the disc minimum thickness is stamped on the hub. And if you can't add 3 and 3 together and subtract that from the reading on an LCD display I'd recommend knitting. A dropped stitch won't matter too much.

No - pay the garage to do it.
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*INDECISION is the key to FLEXIBILITY *

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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Dave Plowman (News) wrote:

I have to say Dave, it's easier to read an inside micrometer than use your method. But neither are rocket science...
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Have you tried my method? It's not compulsory to use it - merely a alternative for the average DIYer who may not have a micrometer but does have a vernier calliper. And in the UK you'll get a vernier caliper in any DIY shed - but not a micrometer.
--
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Dave Plowman (News) wrote:

But the digital readout caliper costs as much as the Cheap-o micrometer and you don't need the allen wrenches or to do any subtraction to figure out what the reading actually is.
I one already owns a digital readout micrometer then sure, go for it.
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It has far more uses than a micrometer. Not as accurate for the things you really need a micrometer for, of course, but a far more useful tool in general. You'd need several micrometers to cover the range of one.
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Usually there's only 2 or 3mm wear allowance so I use my eyes and my fingers.
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You should be a brain surgeon with such sensitive, accurate fingers. But unless you carefully remove the rust etc from the unworn lip and have calculated the amount of wear allowed from new, measuring the actual thickness is the preferred way.
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wrote:

I've had several heated discussions with Dave regarding brakes, but this time he is dead-on.
We have a general disagreement on the best method to measure, and we discuss the merits of machining rotors or not, but we agree that one can not make a valuable judgement of the servicability of a rotor without a measuring device of some sort. If one does not know what the New Condition of the rotor is (in terms of thickness) then one can not evaluate the visible wear to determine if the remaining material is sufficient for future service or not.
What happens if you give a 3mm allowance for wear to a rotor that only tolerates a 2mm wear factor? You would end up with a too-thin rotor, that's what. One has to determine the minimum specification, and ensure that this spec has not been worn through. The only way to do that is with a measuring device.
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All in the best possible taste, of course. ;-)
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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Usually there's only 2 or 3mm wear allowance so I use my eyes and my fingers.
I'm not sure that is a safe practice. The specification is for a minimum thickness, and unless you know the maximum thickness I don't see how you can subtract anything to arrive at a value that is worth while. I don't think you can arrive at a decision as to the servicability of a rotor without physically measuring it. If the rotor is obviously toast, then one would not need to measure, but otherwise a measurment is usually required.
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Jeff Strickland wrote:

It's really not a safety issue. It's a serviceability issue. If the disk gets thin it is more apt to warp. If the disk warps you should replace the pads and disk together because the pads have taken the shape of the old disk surface. So the "idea" here is that it's a false economy to try to squeak more mileage out of a worn brake disk when you are already changing the pads.
My experience is: mos disks that warp do so long before they are worn to the spec. I seldom warp disks late in their life. It seems if the disk is a good one it stays true even after wearing beyond spec. Pads are relatively cheap. I just slap a new set of pads on regardless of if the disk (rotor) is worn or not.
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