View Full Version : Question about rotor size and possibly bad caliper.

07-19-2012, 08:17 PM
I ordered new rotors for my 5 series, just did the 3 series the other weekend. My question is does the rotor size matter? When I placed my order there were two different sizes to choose from, being lazy and automatically assuming that my car had the smaller rotors that were offered, I went ahead and ordered those. To my amusement they were the wrong size, my car has the bigger rotors. I didn't put them on, I went ahead and requested an exchange from the company I ordered from and what not just to be on the safe side. I noticed that the size difference really wasn't that great, but I'd rather be safe then sorry.

About the caliper. When I took the caliper off, I had to push the piston in as usual. When I went to put the caliper back on the piston had come out just enough that I had to push it back in, but it wouldn't reset completely. I didn't want to break the lines to give it some more play because I'll probably be back at it again next weekend and I'm just going to bleed the brakes. Does this mean my caliper is bad and I should replace it or is there something special I have to do to get the piston to slide back and reset? I looked at the price for re-manufactured calipers and it really isn't that bad of a price, so I think I'm going to replace them anyways. My car has 106,xxx miles on it and I'm sure they've never been replaced. How can I tell if it really is bad though?

07-20-2012, 12:01 AM
Are you using a C-clamp or brake tool to push the piston back?

Sounds like you need to relieve the pressure in the line.
Uncap the brake fluid reservoir, place a plastic baggy flat over the reservoir opening and set the cap loosely over the baggy. This prevents fluid from splashing out but allows the fluid to be pushed slightly higher in the reservoir.

The piston should now move back and stay back.

07-20-2012, 07:22 AM
The rear rotors are smaller then the front rotors.

525i front = 324 mm
525i rear = 320 mm

07-20-2012, 08:04 AM
I know the rear rotors are smaller then the front. I didn't order rears thinking they were for the front, but I had two options for size on the front. I ordered the small ones, does it really make a difference?

07-20-2012, 08:05 AM
I had two options for the front 310mm and 324mm. Can I use the smaller rotors that I need to exchange on the back or no?

07-20-2012, 09:11 AM
manual transmission cars require the 310's and automatics take the 324's

07-20-2012, 03:31 PM
alright, thank you. Anyone want to buy rotors for a 525i with manual trans? Cut you a deal and I'll help with shipping.