Abstract: A vent cap system includes a top plate, a center stem assembly, a retainer assembly attached to the top plate, a bottom plate, a flange assembly with a connection end for installation on a suction pile and a plurality of perimeter stem assemblies. The center stem assembly extends from the top plate, through threaded engagement to the retainer assembly, and to the bottom plate. Rotation of the center stem assembly moves the bottom plate between opened and closed positions, relative to the top plate and flange assembly. The perimeter stem assemblies hold the relative position of the top plate to the flange assembly, and the bottom plate seals against the flange assembly, when the system is in the closed position. The bottom plate has a bottom alignment pin to assure alignment of the bottom plate to the flange assembly for any orientation of the vent cap system.