An apparatus for cutting wellbore casing downhole, comprising: a main body comprising a longitudinal bore and a plurality of elongated cutter bases movably attached thereto and each having a tapered nose section on a lower end thereof, said cutter bases attached to said main body by a plurality of positioning arms hingedly connected thereto, said positioning arms being of substantially equal length so that said cutter bases remain substantially parallel to a longitudinal axis of said main body when moved between a first retracted position and a second extended position; a plurality of cutters attached to an upper section of each of said cutter bases, whereby a lower section of said cutter bases without cutters comprises a stabilizer section; an operating mechanism for moving said cutter bases from said first retracted position to said second extended position, said operating mechanism comprising a sliding piston disposed in said bore of said main body and movable by fluid flow through said bore of said main body, said piston having an upper section with a diameter closely fitting a diameter of said main body bore and a seal element between said upper section of said piston and said bore, and a lower section with a reduced diameter, said piston positioned so as to engage a heel section of said positioning arms and bear down on same so as to rotate said positioning arms outwardly and move said elongated cutter bases from said first position to said second position, said piston further having a longitudinal bore therethrough; characterised in that: the lower end of the main body is formed in a rounded point corresponding to the shape of the tapered nose sections, said tapered nose sections comprising a hardened cutting surface thereon. a main body comprising a longitudinal bore and a plurality of elongated cutter bases movably attached thereto and each having a tapered nose section on a lower end thereof, said cutter bases attached to said main body by a plurality of positioning arms hingedly connected thereto, said positioning arms being of substantially equal length so that said cutter bases remain substantially parallel to a longitudinal axis of said main body when moved between a first retracted position and a second extended position; a plurality of cutters attached to an upper section of each of said cutter bases, whereby a lower section of said cutter bases without cutters comprises a stabilizer section; an operating mechanism for moving said cutter bases from said first retracted position to said second extended position, said operating mechanism comprising a sliding piston disposed in said bore of said main body and movable by fluid flow through said bore of said main body, said piston having an upper section with a diameter closely fitting a diameter of said main body bore and a seal element between said upper section of said piston and said bore, and a lower section with a reduced diameter, said piston positioned so as to engage a heel section of said positioning arms and bear down on same so as to rotate said positioning arms outwardly and move said elongated cutter bases from said first position to said second position, said piston further having a longitudinal bore therethrough; characterised in that: the lower end of the main body is formed in a rounded point corresponding to the shape of the tapered nose sections, said tapered nose sections comprising a hardened cutting surface thereon.