A device for root-rolling a thread on an object, comprising: a body having a passageway, and having a cylindrical opening communicating with said passageway; an actuator piston (28A, 28B) mounted on said body for sealed sliding movement within said cylindrical opening, a chamber being defined within said passageway, wherein said chamber is completely filled with liquid; said actuator piston (28A, 28B) having a proximal end facing into said chamber and having a distal end; a thread roll rotatably mounted on said actuator piston (28A, 28B) adjacent said distal end; said body having a first opening communicating with said chamber; whereby said device may be mounted on a machine tool and selectively moved toward said object to root-roll a thread on said object when said object and device are rotated relative to one another. characterised in that the device comprises an accumulator piston mounted on said body for sealed sliding movement within said passageway; said chamber being defined within said passageway between said accumulator piston and said actuator piston (28A, 28B); a coarse adjustment screw threadedly mounted on said body; a resilient member acting between said coarse adjustment screw and said accumulator piston; a fine adjustment screw threadedly mounted on said body in said first opening; wherein the positions of at least one of said coarse and fine adjustment screws (25, 34) relative to said body may be selectively adjusted to controllably vary the fluid pressure within said chamber; wherein said thread roll has an asymmetric configuration; wherein said actuator piston (28A, 28B) has a pocket adapted to receive said thread roll, and wherein said pocket has an asymmetric configuration complementary to the asymmetric configuration of said thread roll such that said thread roll may only be properly inserted into said pocket when said thread roll is in said proper orientation relative to said actuator piston (28A, 28B). a body having a passageway, and having a cylindrical opening communicating with said passageway; an actuator piston (28A, 28B) mounted on said body for sealed sliding movement within said cylindrical opening, a chamber being defined within said passageway, wherein said chamber is completely filled with liquid; said actuator piston (28A, 28B) having a proximal end facing into said chamber and having a distal end; a thread roll rotatably mounted on said actuator piston (28A, 28B) adjacent said distal end; said body having a first opening communicating with said chamber; whereby said device may be mounted on a machine tool and selectively moved toward said object to root-roll a thread on said object when said object and device are rotated relative to one another. characterised in that the device comprises an accumulator piston mounted on said body for sealed sliding movement within said passageway; said chamber being defined within said passageway between said accumulator piston and said actuator piston (28A, 28B); a coarse adjustment screw threadedly mounted on said body; a resilient member acting between said coarse adjustment screw and said accumulator piston; a fine adjustment screw threadedly mounted on said body in said first opening; wherein the positions of at least one of said coarse and fine adjustment screws (25, 34) relative to said body may be selectively adjusted to controllably vary the fluid pressure within said chamber; wherein said thread roll has an asymmetric configuration; wherein said actuator piston (28A, 28B) has a pocket adapted to receive said thread roll, and wherein said pocket has an asymmetric configuration complementary to the asymmetric configuration of said thread roll such that said thread roll may only be properly inserted into said pocket when said thread roll is in said proper orientation relative to said actuator piston (28A, 28B).