Urethane IRS Subframe Bushing Kit, 1979-98

Kit to help swap a 1999-2004 Cobra IRS into a 1979-98 Mustang. Fits the 12mm holes in the 1979-98 Mustang chassis used to attach the IRS subframe.

Improve your Cobra IRS with these urethane IRS subframe bushings. Reduces wheel hop and improves handling by eliminating the deflection allowed by the stock rubber subframe bushings.

I'm new to this; what are subframe bushings?
The Mustang Cobra IRS is a subassembly that bolts to the unibody chassis at 4 points. Each of those bolted joints has a rubber bushing to isolate the subframe from the chassis. Ford did this to reduce NVH (noise, vibration, harshness) transmission from the suspension into the Mustang cabin. Most driving enthusiasts feel Ford was overzealous in its quest to reduce NVH because the rubber bushings allow the subframe to shift its location relative to the chassis. This is bad, leading to wheel hop during hard acceleration and unstable handling in corners.

What's the MM fix?
To reduce wheel hop and improve handling MM designed these hard urethane bushings to replace the stock soft rubber.

Are there any other parts that will help?
Yes. The following items are recommended options (see below, and above to purchase). They're usually used along with the urethane bushings.

• IRS Subframe Bushing Removal Tool: Along with the bushings we designed a tool to help remove the stock bushings. This tool makes the job much easier and reduces the time spent removing the stock bushings by at least an hour.
• 12mm IRS subframe front mounting bolts: The two forward mounting points on the IRS-equipped Cobras have 14mm holes in both the chassis and the steel crush sleeve of the rubber bushings. 1979-1998 Mustangs have 12mm holes for attaching the forward end of the rear lower control arms to the chassis. These same holes are used for the forward mounts of the IRS subframe when it's swapped into the earlier chassis. Problem is, putting 12mm bolts in 14mm holes allows the subframe to shift relative to the chassis. That can cause noise and even changes the rear wheel alignment. This MM bushing kit is designed for the 12mm bolt size of the 1979-1998 chassis.
• Low-profile IRS subframe rear mounting bolts: MM has special bolts for the rearward mounting points of the IRS subframe that feature a lower height and rounded shape of the bolt head. These bolts provide more tire clearance for installing those big rear meats.

Is there anything special about MM's urethane bushings?
Yes. Not all IRS subframe bushings are created equal!

• We use a harder urethane (higher durometer) than urethane bushings made by most others.
• MM's Engineering Team designed these bushings. We do not resell bushings designed by others. Why? The others don't know that Ford had 2 different rubber bushing designs. Their bushings were designed around one of the Ford bushings, so they don't properly replace the other Ford bushing; they just sort-of-fit. The MM bushings were designed with features that let them fit well when replacing either the early or late IRS subframe bushings.

Why urethane and not a harder material?

• The harder the material is the more difficult the installation, because the receiving tubes on the subframe are not round; they are always distorted from the welding done during manufacturing of the subframe. Aluminum or hard plastic bushings require custom fitting all 4 bushings to match each oval subframe tube.
• The multiple mounting points on the 2 welded structures (chassis and IRS subframe) never align perfectly. Trying to slip the 4 mounting bolts into position becomes a nightmare when using very hard bushings.
• The very small amount of deflection MM's hard urethane allows accommodates the oval-shaped bushing shells, as well as the typical misalignment of the subframe to the chassis. What do we mean by "typical misalignment"? At each mounting point, an IRS subframe bolt passes through a hole on the chassis, a tube in the bushing, and a second hole in the chassis. That's a lot of points to line up at just one mounting point! Multiply by four and you arrive at "typical misalignment."
• Urethane bushings remove over 80% of the deflection that rubber bushings allow, which is nearly as much as you'd get from hard plastic bushings, yet without all the headaches that come with using the harder material.a