Sea-Dog stainless steel butterfly vent features a convenient center knob that lets you easily open and close the vent for air circulation when needed. Great for enclosed trailers, smokers, pet boxes, lockers, and more, the vent helps prevent the entrapment of moisture and foul odors. Strong construction is weather and corrosion-resistant. Installs easily with #6 RH fasteners (not included). 4" diameter. 3-5/8" in between mounting holes.