There is provided an insert to be sealingly mounted into a plug hole of a pipe having a sealing area surrounding the plug hole. The insert comprises an internal unit and a fastener. The internal unit and the fastener have corresponding cooperating sealing portions and mounting portions. The sealing portion of the internal unit is formed with a flange adapted to be forced into the plug hole. The flange and the sealing portion of the fastener have corresponding sealing surfaces. These surfaces are adapted to sealingly clamp therebetween the sealing area of the pipe, when the insert is mounted into the plug hole. One of the sealing surfaces being in the form of a sealing edge and the other being in the form of a conical surface. When the fastener is mounted onto the internal unit, the sealing portions of the internal unit and the fastener form therebetween a chamber. The chamber is in fluid communication with the outside environment, allowing atmospheric pressure within the chamber.