The hydraulic Underreamer is a sturdy tool, used to enlarge the borehole beyond bit diameter. It can be opened and closed several times downhole. Run in conjunction with a pilot bit or bullnose. It has 3 large – section cutting arms, each filled with two rows of polycrystalline diamond compacts for maximum reaming. This simple and efficient design, with each arm machined out of a single piece of high strength alloy, ensures longer tool time at bottom and easy field maintenance. Strategically located mud ports provide added cleaning of the PDCs for faster operations with less pack-off risks.