That's normally true, but the 617 uses a high capacity bypass system. Only a small amount of coolant is sent to the radiator, and the output back to the engine is barely warm. When the system is at full capacity, then the bottom of the radiator would be warmer. On my signature car, I can hold on to the return elbow going back to the t-stat housing on all but the hottest days.
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'83 300D, 126K miles.
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