Water proof - "Is a frogs butt water tight"
Someone acting like and idiot - "That person suffers from cranial rectumitis" or simply out "head up their butt".
"Couldn't fight your way out of a wet paper bag."
"6 to 1, 1/2 dozen to another."
Stupid Engineering - "That's like putting screen doors on a submarine"
Precise Measurement - " Closer than a (Pubic) hair
Construction Quality Work - "Looks great, I can't see it from my house!"
"Don't let your bowling ball mouth override your bb brain."