400 (3L80), AT-540, Buick 350, Chevy 350, Powerglide, Powerglide, 350, 400 (258mm) Performance Converter

258mm Performance Converter Kit
Part No. GM-RK-308

View Instructions

Sonnax 258mm performance converter kit GM-RK-308 for GM Powerglide, TH350 and TH400 258mm applications allows builders to assemble a high performance converter with minimal effort. With the readily available GM 258mm converter cores, a variety of stall speeds can be built using different stator and impeller combinations.

  • Fully machined cover with bushing installed
  • Unit-specific design for hassle-free installation
  • Oil flow grooves and holes increase fluid flow to keep converters running cooler
  • Turbine hub features a larger flanged diameter to increase the strength rigidity of the turbine
  • Clear, detailed instructions

Pilot kit not included.

This 258mm kit will not fit GM AT-540 units. View Sonnax 245mm converter upgrades available for this unit.

  • Thrust Bearing
  • Flanged Impeller Hub
  • 30-Tooth Turbine Hub
  • Front Cover, 10-3/4" Single Bolt Patter
  • Front Cover Bushing Installed
  • Unit Size: 258mm
  • Core: 258mm
  • Dampered: No
  • Lockup: No
  • Transmission Unit: TH350, TH400, Powerglide
  • Turbine Hub Input Spline Count: 30

Where to Buy

  • Existing Customers: Contact your sales representative.
  • New Customers: Fill out the form and a sales representative will contact you.
  • 400 (3L80), AT-540, Buick 350, Chevy 350, Powerglide, Powerglide, 350, 400 (258mm) Performance Converter 258mm Performance Converter Kit GM-RK-308

Sonnax Performance Converter Kits

Engineered to Deliver Builds that Last

Designed from the ground up to outperform OE and other aftermarket components, Sonnax has you covered with a full line of performance converter kits for gas and diesel vehicles. Offering the widest variety of options in the aftermarket, these innovative kits deliver customized performance and value.

  • Small/Large Diameter
  • Single-Plate/Multi-Plate/Non-Lockup
  • Dampered/Damperless


Choose the right kit for your rebuild!