Ideal for sealing off an old 4 inch clay pipe used as a rodding eye. The top is hard plastic and is quite robust, whereas the sealing fins are made of more flexible material that makes good contact with the inside of the pipe.
It is early days but it was to replace a plastic cap to a drain on a tarmac surface of a drive. It fits perfectly, is exactly what was described so we now have to see if it is strong enough for cars to drive over.