Email: "Steven C. Morey" <smorey_at_flash.net>
Subject: 750I Thrust Arm Bushing Installation
By popular request, here's a synopsis on installing the heavier 750il thrust arm bushings:
First, the reason that these are needed is simply that the original design is simply too light to "snub" the tremendous forces applied (especially when braking) on the relatively heavy 6 series cars. This is exacerbated by the use over the years of larger and heavier tires wheels with a bushing design from an earlier time that was intended for a lighter 6 with smaller tires wheels. Even so, the original bushing was marginal even for the original 6 series. Although I have not seen a TSB on it, it is my understanding that BMW is now recommending the 750il bushing for all 6 series.
I don't want to elaborate on the fine points of thrust arm removal, this is readily available info. in the Haynes, Chilton, Bentley manuals for the 5 series (which, if you don't know already, is what your 6 body sits on - either the E-12 or E-28 depending on year model). Before you put your car up on four (that's *four*) jack stands, get a huge set of channel locks (the ones that are about 22" long) which you can use to squeeze assorted ball joints while you are under your car to check for play. New thrust arm bushings won't overcome other worn components vice versa.
The 750il bushing is considerably wider than stock (giving more support) and is "denser" rubber. It also has a partial urethane core so they is less rubber "thickness", which imparts a *slightly* stiffer feel. There are some after-market companies that make polyurethane core "performance" bushings that I personally think are counter to what we all love about the BMW feel - that firm yet nicely "damped" feel that filters out high frequency vibrations through the use of rubber. You can buy the 750il bushings pre-milled to fit the 6 series from Dinan for about $180 pair or you can just get the stock 750il bushing and save a bunch by milling it yourself (instructions to follow). I mail - ordered mine from Rod Sydney at Dealer Marketing (I'm sure most of you know him) for about $60 pair.
Whether you are removing the entire thrust arm in order to install the bushing in a press (my preferred technique) or simply dropping the bushing end and using a special tool to do it in the car, get the bushing end out of the chassis mount. Measure the width of the mount opening and the outside width of the stock bushing. They should be pretty close to the same although on mine the bushing was very slightly (.015") narrower. Now, crawl out from under the car and measure the width of the 750il bushing at both the outside (widest point) as well as the "barrel" that houses the rubber. You will find that the barrel is a little narrower than the opening width of the chassis mount (about 0.06"). The objective is to mill equal amounts from either side of the bushing ends until the outside dimension is a *slight* interference fit into the chassis mount. This will still allow the steel barrel to float inside the mount and not make contact with the chassis, thus maintaining the rubber - isolated, damped feel. IOW, when you are finished milling the outside dimension from end to end will still be more than the barrel.
O.K., having said all that, how do you mill the bushing? Simple, really. First, if you just happen to have a nice Bridgeport milling machine in your garage, go for it (you don't need me to tell you how!). If not, you will simply need some kind of power sander, either belt or disc (orbital would take years to remove enough material). If you have a table saw you can install a sanding disc in it and raise it high enough to clear the OD of the bushing. I used a hand held belt sander, turned it upside down (belt face up), clamped it so it couldn't walk away, and locked the power trigger in the ON position. This in effect gave me a stationary sander so that all I had to hold was the bushing. Now simply grind away the soft alloy urethane ends a little at a time, alternating sides. If bushing gets a little too warm, set it aside and work on the other one (you are installing pairs, aren't you?). Don't get too uptight about this, its not that critical. Just check each side frequently for measurement and squareness. I did it all by hand with no problems. If you hate "free-handing" then the table saw cum stationary disc sander will allow you to stabilize the bushing and even use your sliding miter guide to hold it square.
Press bushings back into thrust arms, bolt up to car. Don't forget to *not* tighten the thru-bolt until the weight of the car is on it. BTW, since there is not enough room to do this properly with the car on the ground (i.e. with a torque wrench!). I do it by placing my floor jack under the hub and jacking up each side individually until the car *just* begins to rise off of the jackstand (hopefully you are doing all this with both front wheels removed!). This compresses the strut from the weight of the car to almost exactly the standard ride height, now torque the thru-bolt, unload that side, move to other side of car, repeat.
My M6 had slight braking shimmy prior to this. After checking my rotors (which did have slight run-out) I decided to install the heavier bushings first. Total shimmy elimination. For those of you who have a lot of shimmy, check your rotor run-out, you may need both.
The beauty of the heavy bushing is that they will hold your struts in a more stable position and not allow the rotors to "oscillate" (which begins to amplify and creates even more run-out and rotor warpage). Future rotors will be less apt to warp. Kiss your shimmy goodbye!