Instruction Supplement for Monoball Installation into Control Arms with OEM NON-SMOOTH Bushing Outer Skin

GAS Monoball Instruction Supplement for All Control Arms with Split Rubber or Aluminum Outer Skin OEM Bushings

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

Please follow these additional step if you have an E82 1M, E90 M3, F22, F30, or G29/Supra series or any other control arm that uses an aluminum outer skin 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 Performance Monoball Kit.

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

There are three different types of bushings used in BMW front control arms.

  1. Bushings with smooth stainless steel skins.
  2. Bushings with somewhat smooth aluminum skins.
  3. Split rubber bushings with rough aluminum skins.

Why does this matter? It matters because one of the three versions does not cause any damage to the control arm bore when pressed in and out, and the other two do.

Since stainless steel is much harder than aluminum, a bushing with a smooth stainless steel outer skin can be installed and removed from the control without causing any damage to the aluminum control arm bore. The aluminum of the control arm moves out of the way of the much harder steel skin and no metal transfer takes place.

When you press an aluminum skinned bushing into or out of an aluminum control arm, you have two equally soft metals, and neither can force the other out of the way. What happens is some of the aluminum from both the bushing and the control arm gives way to the other and the end result is metal transfer (galling) between the two parts.

See the following BMW bushing images for examples:

This is an aluminum skinned split rubber bushing from an E90 M3. It is the worst offender out of all of the bushing styles. You can clearly see gouges (a void of metal that transferred from the bushing to the control arm bore), and high spots (a ridge of metal that transferred from the control arm bore to the bushing).

This is an aluminum skinned bushing from an F22/F30 series control arm. It does not show as much metal transfer as the split rubber busing above, but there will be enough damage to the control arm bore that it will need to be addressed using these instructions.

This is an aluminum skinned bushing from an G29/Supra series control arm. It is a little worse than the F22/F30 bushing which also means that the control arm bore damage will need to be addressed using these instructions.

This is a stainless steel skinned bushing from an E60 series control arm. You can see how smooth the outer surface is and the lack of any metal transfer damage on this bushing. That means the control arm bore will look just as good and will not require and repair prior to in stalling the GAS Monoball Kit.

If your BMW control arms use ANY of the aluminum skinned bushings shown above you will need to follow these instructions to repair any damage to your control arm bores before installing the GAS Performance Monoball Kit.

Here you can clearly see the 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 Performance 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 close to a hundred of our monoball kits into control arms with metal transfer damage 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 procedure 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.


We now ship all monoball kits for models that require control arm bore reconditioning with a sanding flap wheel for use with a cordless drill. This replaces the sandpaper sheet that we used to send and is much easier and quicker to use.


Using the high speed setting of your cordless drill, work the sandpaper wheel in a circular pattern around the inside of the bore. Apply firm pressure to the bore surface while making sure that you keep the sandpaper wheel square to control arm and flat to the bore surface.

Work the drill back and forth during this procedure to make sure that you cover all of the control arm bore surface. Typically I run approximately  12 times around the bore surface then flip the control arm over and run the flap wheel around another 12 times with the sandpaper wheel sanding the surface in the opposite direction.


After using the sandpaper wheel 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 wheel 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 run the sandpaper wheel again in a circular pattern around the bore just enough to leave a new sanding 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. You can now finish this process by removing some of the roughness of the sanding finishing with the supplied ScotchBrite pad.

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.