The M-System II "Throwing Star" wheels were standard equipment on several vehicles, including the E34 M5, 540i Sport, and E31 850CSi. The name "throwing star" comes from the 5-star spoke design that is actually a functional attribute of the wheel. The spokes resemble sharp pointy blades and act like turbines that facilitate better brake cooling while the wheels are spinning. Since the wheels on the left and right sides of the car spin in different directions, the wheels would also have to be different to ensure that the wheel functioned equally well on both sides of the car. But instead of making a unique left and right wheel for each E34, E31, and E32, BMW decided to go with a modular wheel constructed from three separate pieces (excluding assembly and mounting hardware). Each wheel has a forged aluminum rim, a plastic hubcap, and a directional wheel cover (i.e. the cover is designed to spin in only one direction).
The forged aluminum rim is lighter and stronger than a cast aluminum wheel, and is the same part # for either the left or right sides of the car. It is available in 8" and 9" widths of varying offset (see part numbers below) and is usually painted black with a silver lip. It is also available with a polished lip. The rim is specific to each BMW model (E34, E31, etc.) and is usually not interchangeable due to offset clearances.
Note: This forged rim is *SOMETIMES* the same part used in the earlier E34 M5 wheel, called the "turbine style" or "whitewall" design because the wheel resembled an old-fashioned whitewall tire. For E34 fitment, older M5 rims may be used behind the newer "throwing star" wheelcovers. EXCEPTIONS: Even though BMW used the same part numbers for all of its M5 forged rims, owner experiences have shown that cars built from 1989 through early 1990 came equipped with wheels of slightly different design. While the overall shape and dimensions are very similar, the face of the wheel's bolt pattern has less clearance for the clips on the plastic hubcap that must fit in the center of the wheel. These clips may have to be removed and other means (adhesive) for attaching the hubcaps may be necessary. (See Drew's Illustrated M5 Throwing Star Upgrade Guide for more information.)
Note: The E34 M5 came equipped with 8" wheels at each corner for U.S. applications, but an optional "Nurburgring package" was available with 9" wide wheels in the rear (see part numbers below).
The second wheel component is the wheelcover. Painted in silver, this lightweight aluminum casting covers the plain black spokes on the rim and serves as the cosmetic and functional face of the wheel. The "throwing star" blades are cast into this part, and five M6 screws are used to fasten the wheelcover to the bare rim behind it (using Loctite 243 Blue on the threads and relatively light torque). The wheelcovers are different for the left and right sides of the car, and when mounting them it's important to remember than the long pointed tip of each spoke should always spin towards the front of the car. The wheelcovers are the same for each BMW model, and M5 covers may be easily substituted on an 850CSi rim and vice versa.
Note: This wheelcover is interchangeable with the older "turbine fan" wheelcovers from the earlier E34 M5 (see above note for rim).
The third wheel component is the hubcap, which is identical at each corner of the car. It is simply a molded plastic piece painted silver, with 5 holes for the mounting bolts and a BMW roundel emblem in the center. It snaps into the center of each wheelcover, and may be conveniently removed when mounting and dismounting the wheels from the car.
Note: The clips on this plastic hubcap may have to be modified to fit on the forged rims from some 1989-1990 E34 M5s (see above notes for rim).
The M-System II "Throwing Star" wheels are popular upgrades for the E34, E31, and E32 models. BMW's 3-piece wheel was an efficient alternative to making unique wheels for every application, and benefits the BMW owner by accommodating so many flexible configurations in a lightwight, performance wheel. However, it is important to remember the proper offset and width clearances for each vehicle, and the factory recommended specs and part #s for every "throwing star" wheel (and associated components) are listed in the tables below.
There are several "Throwing Star" replica wheels on the market today that also deserve some discussion at this point. All of these are one-piece cast aluminum wheels that are somewhat heavier and weaker than the real BMW forged wheels. They are identical on both sides of the car, so on one side they will appear backwards and spin in the wrong direction. They are usually only available in one size (17" x 8" wide) with a generic offset that accommodates many BMWs. The offset might allow an E36 3-series to use the wheel, but the offset will not be optimal for an E34 or other applications. However, these wheels are much more affordable compared to the premium charged for the real deal. An entire set of 4 replica wheels can be bought for $1000, while a *single* M-System II wheel from the dealership can cost you $500 alone!
If I have forgotten to mention any other important details, or if someone wishes to add to this thread, please feel free to reply so that this becomes the best possible archive source for all bimmer owners.
BTW, here's a good example of what the Throwing Stars look like on my car (535iM FOR SALE!):
90 535iM, Sharked, w/ Eibach springs, Bilstein Sports, Eibach sway bars, and Throwing Stars