4 valve K bikes have floating brake rotors in the front. What holds the rotors onto the carriers are "Roll Pins." (a.k.a. "bobbins") These are a wear item that need to be replaced periodically. They will let you know when it's time to replace them. At first you'll hear some slight rattling when backing your bike out of the garage. As they get worse you'll start to hear them rattling when riding at low speed. At a minimum, I'd suggest replacing them when the latter starts to occur but doing it sooner is preferred.
Each rotor is held to it's carrier by 12 roll pins, 9 of them are regular roll pins and 3 of them have an internal spring around them and are equally spaced at every 4th position. The roll pins are #3 in the diagram below. On wheel side of the carriers/rotors are washers(#2) and E-clips(#4) that hold them in place.
There are three approaches to replacing them. The first is to do it with the wheels on the bike. Personally, I wouldn't do that unless I had a bike lift. The second approach is to remove the front wheel from the bike. (The rider's manual that came with the bike shows the proper removal and installation procedures for the front wheel.)
The third approach (what I do) is to remove the front wheel and then remove the carriers from the wheels and replace the roll pins on the workbench. This makes replacing the roll pins very easy and also gives me a chance to get the inner parts of the wheels very clean. If you take this approach, make sure that you are using a good Allen wrench (like the one in the factory toolkit) and that it is well-seated. The carrier bolt heads tend to be a little soft and can strip easily if you are not careful. Using a torch to preheat them or some Kroil (or other penetrating oil) prior to removing them isn't a bad idea either.
I use a small screwdriver to pop the E-clips off. If you look inside the holes where the "springy" roll pins are, you will notice that the spring on the roll pin has caused some wear inside the side of the hole where it mounts. It's a good idea to rotate the location of the springy roll pins by one hole when putting the new ones in.
The regular roll pins go back in easily but the springy roll pins can be stubborn due to the spring around the center. I've found the compressing the spring around the roll pin with small needle-nosed pliers helps get the springy roll pins back in. In order to avoid scratching the rotor or carrier, I use a plastic tool (the rear end of a plastic pen) to push the E-clips back on.
I usually also replace the washers and E-clips but you could probably get way with using the old ones.
Parts required:
Diagram # | Quantity | Part Number | BMW Part Description |
3 | 18 | 34112310086 | Roll Pin |
3 | 6 | 34112310995 | Roller with Snap Ring |
2 | 24 | 34112310088 | Flat Washer |
4 | 24 | 34112310089 | Lock Ring |