On the 617, the thermostat controls flow off the front of the engine and the thermostat measures off the side of the head. Assuming greater convective heat transfer along the whole exterior of the engine, I would think their should be a slight difference, which is what I see too. That being said, you never know for sure if that thermostat is starting to change. I had a heating problem a few weeks ago. The thermostat failed AND the aux water pump failed. I highly doubt one had anything to do with the other. It was just their time.

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'82 300SD - 361K mi - "Blue"

"Good judgement comes from experience. Experience comes from bad judgement."

listen, look, .........and duck.