On a GM 350 transmission (assuming you mean a TH350, as that's the most common 350 transmission), the
smaller diameter line is the inlet and the
larger diameter line is the outlet. This is because the cooler reduces the transmission fluid temperature, and the fluid expands slightly as it heats up. The larger line accommodates this slightly greater volume of warmer fluid.
However, it's crucial to double-check this by consulting a diagram specific to your vehicle year and transmission model. While the size difference is a general guideline, there might be exceptions. Always confirm with a repair manual or your vehicle's service information.
