The rubber plug has an oversized flat washer that prevents it from being inserted too deep- - - - bottom the washer against the outside of the block and tighten the nut. Don't overtorque it- - - -just snug enough to prevent seep leaks is plenty. Don't use any type of sealer- - - - -insert the plug dry. Not sure about the diameter, but I can measure a block tomorrow when I've got enough daylight to see the dial caliper accurately.

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!