The S550's subframe mounting holes are very large compared to the subframe mounting bolts. Since there is a large difference in bolt and hole size, the subframe can be mis-aligned from the factory up to 1/2" left to right. The Steeda IRS Subframe Alignment Kit is made from CNC-machined Delrin to avoid putting two dissimilar metals next to each other, preventing corrosion. Delrin is also less weight then aluminum or steel inserts and therefore keeping performance in mind. The Delrin sleeves slide into the subframe mounting holes, and fill the extra space left between the subframe mount and bolt from the factory. The Steeda S550 Mustang IRS Subframe Alignment Kit guarantees that the subframe will be bolted to the same location every time it is lowered. Eliminating the need for unnecessary alignments.