Bruce said it. Mart said it. Glenn says it.
I first tried this a few years back on a SBC temperature sensor mounted in the intake manifold and it worked wonderfully with no leakage of anti-freeze.
Regarding petcocks I have 2 on my Flathead. I've found that the paddles will spin if they haven't been opened for a while. I soldered the paddle to the splined portion and then JB welded the extension tube to it so that I could slip on a hose to drain in a bucket.
Glenn