GM 6L80 and 6L90 transmissions often suffer failure of the 4-5-6 clutch assembly due to OE design shortcomings. Insufficient clutch piston-to-plate apply surface area causes clutch plate failure, especially when apply pressures exceed OE specifications. The other problem is flexing and bowing of the backing plate (coning), which is a primary source of clutch failure. The heavy duty Sonnax 4-5-6 clutch apply piston kit 104960-01K addresses these weak areas in two ways: A modified OE clutch apply piston and piston dam are used in conjunction with a specially-designed Sonnax piston apply ring. A more rigid backing plate. The Sonnax apply ring increases surface area between the apply piston and OE waved clutch plate, resulting in more even distribution of apply force to clutch plates. The Sonnax backing plate minimizes flexing/bowing when clutch is applied, drastically reducing the likelihood of clutch plate distortion. Solid backing plate design outperforms OE and aftermarket plates Increased apply ring surface area strengthens and reinforces assembly Contents Apply Piston Apply Ring Piston Dam Backing Plate