Sonnax Smart-Tech drum module for big main shaft 34555-03K upgrades racing GM TH400 transmissions previously built with Coan Racing XST 1-1/8" main shaft. This unique Sonnax drum system is proven to improve on-track performance and consistency, leading to quicker ETs, more wins and greater reliability. The big main shaft version features a forward clutch hub, sprag shaft and sprag-shaft-to-center-support-bearing adapter configured to work with the Coan shaft.

The TH400 is preferred for its overall durability and multitude of available ratios, but is known to be heavier and less efficient than the popular two-speed Powerglide. Part of the TH400 inefficiency results from the fact that the direct drum counter rotates, spinning backwards in 1^st Gear, brakes to a complete stop on the 1-2 shift, then goes from 0 RPM to input RPM on the 2-3 shift. Significant energy is wasted through this cycle that robs power, upsets traction on the shifts and contributes to broken internal parts.

Compared to aluminum direct drums, the Smart-Tech drum module reduces the weight of the counter-rotating components by over 3.3 lbs! And more significantly, because the weight of the Sonnax counter-rotating parts is closer to the centerline, there is a 53% reduction in energy required to stop and restart during the 1-2 and 2-3 shifts. The benefits are increased wheel horsepower, better acceleration, less shock on the shifts and improved durability.

• Kit replaces both forward and direct drums and includes everything from intermediate sprag to the pump, minus frictions and steels
• 100% new content, no used core pieces
• Includes larger-diameter, 36-element sprag and sprag race that works with OE intermediate clutches