Fluid plug

A downhole tool, comprising:

• a housing having an internal chamber, a port having a first portion with at least one internal thread and a second portion in communication with the internal chamber and having a reduced diameter portion defining an annular surface;
• and
• a plug positioned in the port and having a disk with an upper surface, a lower surface and an annular groove in the lower surface, the annular groove having an inner wall, a first outer wall, a top and a bottom, the first outer wall of the annular groove being tapered radially inwardly from the bottom to the top of the annular groove, an elastomeric annular member positioned in the annular groove, an annular member positioned in the annular groove between the elastomeric annular member and the first outer wall and having second outer wall, the second outer wall being radially inwardly tapered to establish wedging engagement with the first outer wall of the annular groove whereby the annular member is retained in the annular groove, and a member having at least one external thread for engaging the at least one internal thread of the port whereby rotation of the member in a given direction moves the disk toward and compresses the elastomeric annular member against the annular surface.