The OE Powerglide servo design is inherently flawed. Its spring-powered design suffers from an improper apply/release ratio that is responsible for otherwise unexplained band failures, input shaft failures and staging issues. Now known as the pin bias effect, the resulting burnt bands, broken parts and inconsistent operation have plagued the Powerglide for years, leading many to give up on this transmission and seek other options.
Sonnax Smart-Tech® ratio-style servo kit 28821-10K is a revolutionary part that conquers the pin bias effect once and for all, making this the ultimate choice for running Powerglides in ever more extreme applications. The unique design has the same apply area as the OE Powerglide servo, but increased release area so the servo is hydraulically released like other modern servos such as the C-4, C-6 and 4L60.

All-in-one kit makes it easy to install a state-of-the-art servo system
Supported servo design eliminates piston binding
Significantly lighter spring prevents staging issues and backing up when trans brake is applied
Prevents input shaft failure due to tie-up on shift
Allows greater tuning of high clutch apply to prevent tire spin and pedaling after the shift
Delivers more total net apply force than any stock-sized servo
Eliminates need to calibrate servo return spring to line pressure or varying case depth
Improved sealing throughout reduces fluid loss and is optimized for low friction movement
Low-profile design ensures adequate clearance
6061 Anodized billet aluminum cover