I think I have found the plug to remove to drain the block on my 6 cylinders, but how do I remove it? Looks like a pipe thread.
Note: noticed "black soot" in my overflow tank.
Thanks.

 

Not sure what plug you're looking at, but I always just pull the lower radiator hose.

 

Bingo! I do the same. The lower radiator hose is easy to get to and drains quickly.

Also, Lisle makes a water gun that has a barbed fitting on the end of it. Great for flushing via the heater hoses.

 

if it's small black particles, it maybe your hoses deteriorating. Perhaps time to change them.

The block drain is on the driver side of the block just behind the motor mount. It takes a square drive end like the ratchet (no socket).

I wouldn't pull it out. Like the others said, I just pull the lower rad hose. Yes some will be left in the block. I just refill with water, let it run a bit, and repeat the drain. Again if needed until the water is clear.

I also use my garden hose to flush the lines, core and passages as best as I can. Be aware that city water pressure is pretty high so don't go full stream.

Try to catch as much coolant as you can in a pan/bucket during your initial drain. Then take to hazardous waste. Do not pour down a storm drain or dump on the yard, woods, etc.