Supported Servo Master Kit
Part No. 28821-20K

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Conventional GM Powerglide servos are not well-supported and the return springs do not function properly, both leading causes of band failure. Sonnax supported servo master kit 28821-20K is an all-in-one kit that eliminates piston binding and includes a race-calibrated spring to counteract the pin bias effect.

At high apply pressures or with a worn servo pin bore, the servo piston is side-loaded in the case, causing friction and binding of the piston. The supported pin design of this Sonnax kit eliminates friction caused by side-loading and ensures the servo piston will apply and release without binding.

"Mystery" band failures are a common result of aftermarket servo return springs and weak OE springs prone to collapse that do not reliably release the servo. The Sonnax spring in this kit is race-calibrated to ensure consistent servo function for longer band life and a cooler transmission. The spring's higher seat pressure helps ensure full servo release, but also features a lower spring rate to prevent excessive release force in the apply position.

  • Unique cover and pin combination eliminates piston binding that causes unexplained band failure
  • Race-calibrated spring counteracts pin bias effect up to 275 psi
  • Dual PTFE seal rings for better seal and low friction
  • OE apply area
  • Low-profile design ensures adequate clearance
  • 6061 Anodized billet aluminum cover
  • Supported Servo Cover
  • Supported Servo Pin
  • Servo Piston
  • Return Spring
  • Retaining Clip
  • Gasket
  • Cover O-Ring
  • Pin O-Rings (2) 1 extra
  • Drain Plug Not shown
  • Bolts (3) Not shown
  • Pin Shims (3)
  • Spring Shims (2)
  • PTFE Piston Seal Rings (2)

  • Powerglide Supported Servo Master Kit 28821-20K
  • Powerglide Supported Servo Master Kit 28821-20K details


Prevent "Mystery" Powerglide Band Failures by Overcoming the Servo Pin Bias Effect