For me, the cleanest and easiest way to drain a controlled amount of coolant is from the radiator drain ****, which is located near the radiator outlet hose.

 

I had a similar experience with my temp gauge on my 68 BB earlier this year. First, when changing the sending unit I let the car cool, prepared the new sending unit for installation (teflon around the threads only), loosened and quickly removed the existing unit with the new one inserted. I couldn't of lost more the a cup of coolant. Secondly my problem was the I installed the gauge pigtail upside down, ie, the green (gauge) wire on top. My symptoms were quite similar to what you describe. My current gauge is reading high, 210 when the real temp is 190 (laser gun). I'm still trying to decide what I am going to do with it.

 

Ok... I'm a little frustrated. I installed the new temperature sender and 195 degree thermostat today, but the problem remains. The needle on the gauge does not pass the first mark. I did test the thermostat before the installation, but I forgot to check the new sender.

I after installing the new parts, I let the car idle for about 20 minutes. The needle only went to the mark after 100 degrees. Then I took the car on a drive for a few miles in local traffic and never went above third gear. The needle still didn't move any further. There were a couple times when the needle moved forward the slightest bit, but only for a few moments.

I used the temp gun when I got home and took two readings. The gun read 225 degrees when I aimed it at the sensor and 186 degrees when I aimed it at the thermostat housing. I also test the old sender in a pot of boiling water. After several minutes suspended with only the bottom part submerged, the lowest it dropped to was 93 ohms... which according to the info above, shouldn't even have moved the needle on the gauge to the first mark.

So, here's what I'm thinking:

1. Since the gauge seemed to do what I expected it to do when I hooked up the 100 ohm resistors from Radio Shack, I might need the adjustable resistor.

2. I may have the wiring on the back of the gauge upside down.

3. Since I do not have a Corvette engine, my new sender might still be the wrong one. The engine block code is V0926TAX which tells me the block was out of a 1970 or 1972 pickup truck. Unfortunately, I've spent a couple hours trying to find the date code without any luck.

At this point, I think I'll pull the gauge cluster and see how the wires are hooked up. If they're good, I might as well leave the cluster open and install the adjustable resistor.

Thoughts?

 

You should be fairly easy to verify the pigtail's direction. I was able to access my temp gauge pigtail by removing the right side dash. In fact I had a spare gauge the temporarily hooked it up only to have the wire upside down, green wire on tip. The wiring diagram showed it that way and it was 15 years earlier that I took the gauge cluster apart.

 

That sounds much more appealing than removing the cluster!

 

Success!

Everything was hooked up properly behind the dash, so I pulled out the new sender from Willcox and hooked it up to the multimeter. In boiling water it read 100 ohms when our gauge needs it to put out 79-82 ohms. I can see why you would need to purchase the adjustable resistor if you want to use this sender.

Then I came across this post and picked up a sender from Advance Auto. The sender doesn't look like stock because the terminal is the thumbtack style, but it puts out 84 ohms in boiling water which means the resistance is close enough to make it work without any additional parts.

After letting my '72 idle for about ten minutes, the gauge read a hair below 210 degrees. While out on a 2-hour ride in 80 degree heat, the gauge fluctuated between a little over 210 degrees and the second mark. It usually stood right between the two. I'd like the engine temp to be closer to 210 degrees throughout my whole ride, but that will be a project for another day. Right now, I'm just going to enjoy having a working gauge.

Thanks to everyone who provided advice. I really learned a lot throughout this thread.