GM 6L80 and 6L90 transmissions often suffer failure of the 4-5-6 clutch assembly due to OE design shortcomings. Not enough clutches and 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.
Sonnax high capacity 4-5-6 clutch apply piston kit 104960-20K features a modified OE clutch apply piston and piston dam that are used in conjunction with a specially-designed toothed apply plate closest to the piston. The Sonnax toothed apply plate increases surface area between the apply piston and first friction plate resulting in more even distribution of apply force to clutch plates and allows a friction plate to run against the apply plate. This configuration eliminates OE waved plate. Both the toothed apply plate and the backing plate have a smaller inner diameter with wider friction surface intended to be used with Powerglide-style frictions, special steel plates and aftermarket 45-tooth 4-5-6 clutch hub.