Circle D GM 258mm Pro Series PG/350/400 Torque Converter

Circle D Specialties GM 258mm Pro Series PG/350/400 Torque Converter

The Circle D Specialties GM 258mm Pro Series PG/350/400 Torque Converter is a premium, high-performance upgrade designed for heavily modified street and strip vehicles equipped with a GM Powerglide, TH350, or TH400 transmission. The 258mm (10 1/4-inch) diameter offers a perfect middle ground—significantly reducing rotational mass for rapid throttle response and quick acceleration, while maintaining slightly more surface area than our 245mm option for excellent streetability. Featuring a versatile screw-on pilot, this converter can be easily adapted for use with classic early GM engines as well as modern LS and Gen-V LT powerplants.

Engineered to survive heavy track abuse, this Pro Series converter features a rigid custom CNC-machined billet steel front cover to eliminate flexing under load. The internal assemblies are fully furnace brazed and hand TIG welded for maximum structural integrity, while an upgraded 3-bearing setup ensures smooth, reliable operation under massive torque loads. For builders running heavy nitrous or extreme boost, an optional Heavy Duty Upgrade is available, which adds essential additional impeller balloon plating and heavy-duty welding for absolute bulletproof reliability.

Key Features & Upgrades

• Designed for GM Powerglide (PG), TH350, and TH400 transmissions
• 258mm (10 1/4") diameter delivers rapid throttle response with excellent streetability
• Innovative screw-on pilot allows adjustability for early GM or modern LS / Gen-V LT setups
• Rigid custom CNC-machined billet front cover prevents flexing and ballooning
• Optional Heavy Duty Upgrade: Adds additional impeller balloon plating and heavy-duty welding
• Fully furnace brazed with hand TIG welded construction
• Premium 3-bearing setup for maximum durability and reduced friction
• Hand-assembled to exacting tolerances for reliable, dominant track performance

Specifications

• Application: High-Performance Street/Strip Torque Converter
• Transmission Compatibility: GM Powerglide, TH350, TH400
• Converter Diameter: 258mm (10 1/4")
• Construction: Custom CNC Billet Front, Furnace Brazed, Hand TIG Welded
• Bearings: Upgraded 3-Bearing Setup
• Options: Multiple Stall Speeds (2800 up to 4000 RPM), Sprag Options, and Heavy Duty Upgrade available

Notes

• Custom manufactured product built to your exact specifications.
• Estimated lead time: 3-4 weeks.