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Race-Calibrated Servo Return Spring Kit
Part No. 28133-HD

OE Part No. 3830235, 3815914, 3830235

View Instructions

Mystery band failures in GM Powerglides are a common result of using inconsistent aftermarket servo return springs and weak OE springs prone to collapse. This is a problem that can be traced directly to the pin bias effect. Sonnax race-calibrated servo return spring kit 28133-HD counteracts pin bias effect 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.

Watch this video to see exactly how an OE servo performs at high pressures when fitted with an OE, light aftermarket or Sonnax race-calibrated spring.

  • Counteracts pin bias effect up to 275 psi
  • Includes two .050” shims to aid in tuning at higher line pressures
  • Works with both OE and aftermarket servo pistons
  • Also available in Sonnax servo kit 28821-20K
  • Shims (2)
  • Spring

April 27, 2016

Understanding & Overcoming the Powerglide Servo Pin Bias Effect - Part 1

Topics: Diagnostics And Theory Transmission GM Powerglide Transmission

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April 26, 2016

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

Topics: Diagnostics And Theory Transmission GM Powerglide Transmission

Watch Now
  • 28133 hd in