The
metal flap which covers the exhaust on larger trucks and tractors. Often known as an exhaust flapper which prevents
water from entering the engine and exhaust
system when not in use.
When the engine is in use, the flap or "pub" pivots up and down allowing the exhaust gasses to safely escape.
The hick down the street put a set of exhaust stacks on his
F-350 and it even has a set of pubs now!! Every
time he starts his
truck up the little pubs move up and down.