The block drain is at the bottom of a V-cavity, so it isn't very deep there, just enough clearance for the plug. When you remove it, coolant usually doesn't flow out until you break up the rust crud w/ a screwdriver. A stagnant area which won't measure typical coolant temperature. For sure don't screw one in, bottom out, and strip the threads or crack the block. The plug is 1/8" NPT in a V-8 (one on each side) and 3/8" NPT in a slant (single).
