Rear subframe cracks for years have been a sore subject among E46 M3 enthusiasts, the failures are common enough to say its not a matter of if, but when. BMW has tried to address the failures over the years with a number of service campaigns that include additional bracing or even injecting foam reinforcement, these fixes are temporary at best. The best solution to this failure is to weld any existing cracks to prevent them spreading further and to weld in place thick metal reinforcement plates in the areas of highest stress. A large number of companies produce a reinforcement kit, in our experience we have found that Achilles Motorsports produces the highest quality and strongest kit. Their rear subframe reinforcement kit consists of eight 1.85mm weld in pieces of steel which distribute the load from the subframe over a larger area on the body. Even if your particular E46 M3 is not exhibiting subframe cracks we still recommend this kit as a preventative maintenance item that will save money and headaches in the future.
Below in this article we have documented the repair process for all to see. This repair requires extensive disassembly of the entire rear of the vehicle and expert welding skills. Due to the extensive nature of the repair this is not something Parts Score recommends that is done as a DIY. The exhaust, driveshaft, differential, gas tank, suspension, and rear subframe all have to be removed to access the location of the cracks. Once the car is disassembled the body is thoroughly cleaned and prepared for repair. The cracks are filled and then the reinforcement plates are welded to the body. Afterwards a coating of sealant paint is applied to prevent any moisture from entering and causing rust. Once this is completed the M3 is put back together and is back on the road!
As we mentioned due to the extensive disassembly and professional welding skills needed this is not a repair we recommend you attempt at home. We have done a large number of these E46 M3 subframe reinforcements and expect to see more in the near future as the E46 M3’s rack up the mileage and go on to have second, third owners, and even fourth owners. If you have an E46 M3 contact us now to schedule an inspection and if any cracks are found we can get you scheduled for repair. Once cracks start to form the transformation from small issue to your subframe ripping out can happen rather quickly especially with spirited or track driving.