Tool made for the purpose, available where automotive tools are sold:

 

I reach down from the top to loosen the drain plug.

 

It's much more difficult on the SS.

I think FillerNeckSupply.com has a drain that goes inline with the lower hose. I'd link but their site is down for maintenance right now.

 

There are many sources for the drain , however I don’t like the idea of cutting a perfectly good hose and add 2 more clamps to possibly leak. Plus the lower rad hose isn’t at the bottom of the radiator tank


This version was from Jegs website

 

I bought a Mahle TX12282, along with a GM Genuine 131158. Looked identical. Tested them in a pan of heating water, and they opened in lockstep.

At Rock Auto the Mahle is $15.42 and the GM $66.79. Betcha Mahle made them both. Mahle is an OEM supplier of thermostats.

 

Interesting

 

Opening and closing is not the usual problem. It is the inner rubber gasket failing and jamming the valve open.

 

Yeah that appears to be what happened in my case. Drove it tonight so far so good👍

 

I wish i knew that before I spent $66 dollars on an oem stat

 

I’ve installed the Mahle thermostat and it failed after a month and a half