Instruction Supplement for TRW Split Bushing Monoball Installation

GAS Monoball Instruction Supplement for TRW Control Arms with Split Rubber OEM Bushings

This instruction supplement covers repairing damage to control arm bores caused by aluminum skinned, split, solid rubber factory bushings. These repairs are necessary to allow for proper fitment of your GAS Monoballs.

Please follow these additional step if you have an E82 1M, E90 series M3 or any other control arm that uses a split rubber factory bushing. These control arms suffer damage to the bushing bore I.D. when the bushings are pressed in at the factory and again when they are pressed back out to install your GAS Monoball Kit.

Perform these instructions only in a very well lit environment. Take your time and don’t rush thru it.

If you have an E82 1M or E9X M3 your BMW should have been equipped with TRW brand control arms with a split rubber bushings that look like this.

In the picture on the left you can see the gouging and metal tranfer that was the result of the bushing being pressed in at the factory and pressed back out in preperation to install your monoball kit.

Here you can see the similar damage to the inside of the control arm bore. The damage consists of low spots (gouges) and high spots (metal transfer). The low spots are not a problem but the high spots should be removed for proper fitment of your precision machined GAS monoball kit.

If the control arm bores are properly repaired you will not experience any metal transfer or gouging of your new GAS monoball housings when you install them. This allows you to press them out and reuse them in the future if you need to replace your control arms.

After installing dozens and dozens of our monoball kits into new TRW “M-series” control arms I have found that the following procedure works well without the need for specialty equipment. Perform these instructions in a well lit area and take your time. The precedure is not very difficult, it just takes a little time and care.

Shown here are the two files that I use to start removing the high spots from the control arm bores.

These files are flat on one side and rounded on the opposite side. They are typically referred to as “half round” files. These are small files called “needle” or “jeweler’s” files with a file section approx 4 inches long. I start with a medium cut file and finish with a fine cut. You can do the entire job with either if you don’t have both.

If you don’t have a needle file set and do not wish to spend much money this set is available from Harbor Freight for $2.49.

This set does not include a “really fine cut” half round file like the second one shown in these instructions, but the medium ones should be fine.

Harbor Freight does not list part numbers on their website but this file set is listed as “Central Forge Needle File Set, 12pc”.


Start with a medium cut half round file and carefully start removing the high spots. Work the file at the angle shown in the picture.

The goal is to just bring the high spots down to flush with the bore surface, not to enlarge the bore diameter.


It’s probably difficult to really see what was done, but this picture shows the control arm bore after using the medium cut file.


Next I repeat the procedure with the fine cut half round file at the same angle like what was shown for the medium cut file.  I also work the file in a crisscross pattern as well. Again, the goal is to only remove the high spots, not enlarge the bore.

The picture at left shows the control arm bore after using the fine cut file.


To check for remaining high spots use a utility knife blade held 90 degrees to the bore as shown. Drag the blade back and forth around surface of the bore sliding the blade sideways as shown in the picture (not the cutting direction). High spots will cause the blade to catch as you slide back and forth. BE CAREFUL HANDLING THE BLADE TO PREVENT CUTTING YOURSELF.


Next I use 180-220 grit sandpaper. I use 1/4 of a standard full size sheet, then fold that piece twice so that you end up with a 2.75″ x 2.25″.

This give you usable size piece that is rigid enough to hang onto and also allows you to flip it over once the first side gets worn out or loaded up.


Work the sandpaper in a circular pattern around the inside of the bore.

Those of you with a sharp eye may have noticed that this is not the same control arm that was shown in the other pictures.  I forgot to take this shot during the original photo session and had to come back and take it later. I didn’t have a naked TRW arm on hand at that time.


After using the sandpaper look closely around the inside of the bore. Low spots will appear as dark lines, and any remaining high spots will appear as light colored islands with a thin dark line surrounding the perimeter of the island. If you see any of these islands remaining, use sandpaper or a fine file working just the high spot. You can see examples of that in the picture on the left.

After working the high spot islands run the sandpaper again in a circular pattern around the bore just enough to leave a new patter so you can check again for high spots.


Once the high spots are gone your control arm bore should look like the control arm shown in the picture. This is a finished bore ready for monoball installation. If you performed this procedure correctly your monoballs should require minimal pressure to install.

You can now close this window and continue on with the main monoball installation instructions.