Hydraulic operated underreamer (1) for enlargement of boreholes. The underreamer comprises a cylindrical body which is arranged for connection to a rotating drill string. The reamer (1) has an outer movable cylinder (2) which is stretching out in the whole length of the reamer for protection of the inner parts. The reamer has cutter support arms (5) which in operative state are extending out from the body and in closed state are completely retracted within the cylindrical body. Cutters are mounted on the support arms (5), which are axially movable. The outer cylinders (2) have openings for passage of the support arms.
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