[mitchntx]

Glenn has done tons of research on this and we are both about to the point of have custom machining done.

Autometer guages have a sending unit that is 1/8" NPT.
An LS1 motor has ports machined in, but are 12x1.5 metric.

Sure there are adaptors, but the wall is so thin the adaptors twist off with MINIMAL torque. And by that, not enough to crush a seal enough to prevent a leak.

Why don't they make a 12x1.5 sender?????

The stock sender ohms out WAY different ... almost 1K ohms lower than the AM.

[Cal]

and just for an FYI - i said "F" it, and pulled the water pump, drilled a hole just to the left of the T/B on the round part of the water pump that faces up. tapped it 1/8" NPT and put a temp sender there.

I use an Auto Meter Cobalt full sweep water temp gauge and love it; best of both worlds: full sweep like a mechanical gauge, yet the ease of installation of an electric gauge. This gauge is actually more "electronic" than "electric" since it uses a microprocessor to read the sensor, then drives the needle with a stepper motor.

I got the necessary adapters from Auto Barn, however I didn't use them. I first tried the water pump location, but didn't like that because you don't get a temp reading until the T-stat opens. So I moved the sensor to the head, but did not use any of the normal ports. Instead, I took out one side of the bubble tube, then drilled and tapped the hole for the size of the sensor itself. This way I didn't need any adapters, plus the sensor is in the head for the best readings. I can post some pics if anyone is interested.