The TH350 and TH400 are the two automatic transmission choices for classic C10 builds. Both are bulletproof, but they serve different purposes.
TH350 – Light, Efficient, Easy to Find
The TH350 is a 3-speed automatic that was standard behind most V8 small blocks in the 60s and 70s. It’s lighter than the TH400, shorter overall length, and less parasitic drag. For street-driven trucks without big power goals, it’s the right choice.
It holds up well to 450–500hp with a mild rebuild and good torque converter.
TH400 – Built for Torque
The TH400 is a heavy-duty 3-speed that was designed for big block applications and trucks with high load demands. It’s nearly indestructible, handles 600+ hp in stock form, and is the go-to choice for LS swaps, performance builds, and towing rigs.
The downside is weight — it’s about 75 lbs heavier than a TH350 — and slightly lower efficiency.
For Classic Trucks:
- Daily driver with stock or mild power → TH350
- LS swap, big block, towing, or spirited driving → TH400
- If you want overdrive, consider a 700R4 (direct TH350 swap) or 4L80E with adapter
The COMP Cams Big Mutha Thumpr cam kit pairs well with either trans for a street performance setup.