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I actually have 2 or 3 infrared guns laying around.. I'm a HVAC contractor and have quite a bit of knowledge of heat rejection etc.. I use them on the Cobra all the time (checking individual header primary temps helps with adjusting 4 corner Holleys and Barry Grant carbs)

The temp gauge appears to be operating correctly all the rest of the time so I suspect it's working correctly when it's reading 250 deg. There's no potential difference from the sending unit's housing to ground too (good ground contact). It's conceivable that there's a 250 deg pocket of steam flowing past the sending unit while the rest of the system's not up to temp. That would account for the disparity between the gauge reading and the "feel" of the intake. I'm thinking (hoping) that the 4ea 1/4" holes will take care of the steam pockets. If they don't then I need to rethink my problem solving tree and look elsewhere for the problem/solution.. I'm banking that's not the case. The head gaskets are fine (I assembled). If they weren't I'd be overheating all the time. The gauges are electric, not cap tube. All the Robertshaw (Milodon and Mr. Gasket) high flow thermostats (BB Chrysler and the smaller current style) have the same diameter openings, so there's no advantage there as I'm using the Robertshaw now. I'm running Water wetter too. You need to stay away from tap water (I used distilled water) as it contains too many conductive minerals which promote galvanic corrosion and electrolitic activity.

Coolant recovery kit??

Thanks for the help/suggestions,
Dave