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General E46 Forum
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Old 10-25-2008, 04:41 PM   #1
ct126x
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brakes

my brake pad light went on today. How do I tell if I need to replace the rotors? Should I replace just the fronts or do I need all four rotors and pads?
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Old 10-25-2008, 08:48 PM   #2
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Found this on another forum,

"New rotors are required when the old ones are too thin and/or rusty (inside where the vents are). A bit of rust is normal; heavy flaking means it's time for new ones).

You need a micrometer or decent machinist's calipers to accurately measure the rotor thickness. The minimum thickness rating is usually stamped on the rotor itself.

Every new set of pads MUST also have either new rotors or machined rotors. I would suggest removing the old rotors and taking them to a local mechanic and ask if they can be machined. They can measure them and tell you if they are too thin or rusty or if they can be machined. Around here the cost for machining is about $20 each.

You may find new rotors are so cheap that machining does not make much sense. For example, my '97 DeVille's rotors cost me $26 new. Therefore, it did not make sense to drive out of my way to get the old ones machined for $20. The six bucks was not worth the time and effort just to reuse old rotors.

As for the rear brakes, pull off the wheels and have a look at the rear pad thickness. If the lining is less than half the thickness of new brake lining, its a good idea to do them."
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Old 10-25-2008, 09:14 PM   #3
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mine went off maybe a month or two ago. i ignored it and i went to the shop today, and they said my rear pads are just about metal on metal. go to a shop and have em check your pads, most will do it for free.
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Old 10-25-2008, 09:24 PM   #4
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It depends on how rich you are. If you are very rich, you can replace all 6 sets of rotors and pads. If you are just doing well, you can replace the 4 sets of pads and rotors. If you want to save a bit, learn to measure the rotor width and only replace them if necessary. Shops will always want you to replace everything, which is understandable as it is only your money.
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Old 10-25-2008, 11:59 PM   #5
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The reason BMWs brake service policy is to replace pads and rotors is to reduce the risk of squealing brakes. Apparently the surface of the used rotor has unevenness that matches the old pads. When new pads are put on, the rotor surface can cause squealing. Depending on the time cost of redoing brake work, its probably a good choice to buy a set of rotors too. Anti-squeal compound only addresses caliper to pad contact.
Also, if you want any shot at reusing the brake sensor, you need to change the pads immediately because the electrical contact wears off along with the remaining few mm of brake pad.
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Old 10-26-2008, 12:05 AM   #6
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On average, OEM pads run 15-25k miles, replace rotors every other pad replacement. Bleed and replace brake fluid every 2 years. SS brake lines help too. Always replace the sensors 1 front L, 1 rear R. Parking brake adjusted/relined every time you to the rotors.
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Old 10-26-2008, 02:31 AM   #7
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It depends on how rich you are. If you are very rich, you can replace all 6 sets of rotors and pads. If you are just doing well, you can replace the 4 sets of pads and rotors. If you want to save a bit, learn to measure the rotor width and only replace them if necessary. Shops will always want you to replace everything, which is understandable as it is only your money.
theres six of them?
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Old 10-26-2008, 03:49 AM   #8
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theres six of them?
No there is only 4. That guy typo'ed. Most likely it's your front brakes that need to be changed, since they do the vast majority of the stopping. The only clear way to check is to take the wheel off the front and rear to see which one the sensor is indicating needs to be replaced. You can get a full OEM front or rear kid from Tischer getbmwparts.com for $200 per axle. It's a great deal. Make sure that the shop you take it to is using OEM parts, or better yet do it yourself and save a few hundred dollars. It is advisable to use change the rotors when changing the pads and make sure you change the sensor ($10) , because once the indicator is tripped it shorts out.
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Old 10-26-2008, 10:47 AM   #9
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theres six of them?
Yes, six sets for the rich. If you have to ask, then you are not rich.
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Old 10-26-2008, 10:55 AM   #10
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Rotors and pads should always be changed at the same time.
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Old 10-26-2008, 11:05 AM   #11
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^ this is usually what we do at work. but honestly, they can be resurfaced, but its not as good as replacing them
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Old 10-26-2008, 11:06 AM   #12
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^ this is usually what we do at work. but honestly, they can be resurfaced, but its not as good as replacing them
Only if the rotors haven't developed that big of a lip yet or if they're just slightly warped. Otherwise, you're just thinning down the metal too much.
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Old 10-26-2008, 11:09 AM   #13
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tis true... but the size of the lip don't matter if you can resurface them and still be within spec. but yes...the best thing to do is replace rotors with the pads...but you don't HAVE to. I put front pads on my car and rotors were within spec without a lip or warpage...and i've been drivin on them for about 10k so far without any squeaks at all
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Old 10-26-2008, 11:12 AM   #14
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It definitely isn't required, but is recommended because if you replace one and not the other, the more worn part can damage the new one. Warped rotors or rotors with lips in them can cut into new pads, and worn pads can cut into new rotors.
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Old 10-26-2008, 02:43 PM   #15
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im ordering from tischer right now. do the rotors and pads come in pairs? or do i need to order 2 front rotors, 2 front pads, etc?
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Old 10-26-2008, 03:13 PM   #16
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usually come in pairs
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Old 10-26-2008, 03:45 PM   #17
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It definitely isn't required, but is recommended because if you replace one and not the other, the more worn part can damage the new one. Warped rotors or rotors with lips in them can cut into new pads, and worn pads can cut into new rotors.
You're talking about the lip on the outside edge of the rotor? That should be machined off if you're re-using the rotor. As long as the rotors after machining are not under the minimum thickness, it's perfectly fine to re-use them.
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Old 10-26-2008, 04:07 PM   #18
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so there is one sensor in the rear, and one in the front?

i know how to replace the rear pads, but i'm not sure about the sensor. is the sensor a separate part i need to order?
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Old 10-26-2008, 04:25 PM   #19
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so there is one sensor in the rear, and one in the front?

i know how to replace the rear pads, but i'm not sure about the sensor. is the sensor a separate part i need to order?
If the sensor wasn't tripped (light on cluster) you don't need to replace them. There is one on drivers side front and passenger side rear iirc
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Old 10-26-2008, 05:19 PM   #20
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Why does everyone always say replacing the rotors with new pads?

This seems like a serious waste of money. Rotors usually last at least twice as long as pads, and there usually isn't such a thing as warped rotors, usually just uneven pad deposits. This happens like when people drag their brakes down a long hill or hard braking getting off a freeway and come to a complete stop, leaving the brakes pads and rotors just resting against each other while they are still hot. People should go try properly bedding in their brakes more often since this burns away old pad material...

Also, for various reasons, BMW rotors should not be machined. Either they are still useable, or need to be replaced.
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