A hydraulic torque impulse generator comprising a drive member (10) drivingly connected to a rotation motor and formed with a fluid chamber (11), and an output spindle (12) extending into the fluid chamber (11). Two radially movable seal elements (14, 15) of cylindrical shape are supported in a through slot (13) in the output spindle (12) and are arranged to sealingly cooperate with two axially extending ridges (17, 18) in the fluid chamber (11). A first set of two diametrically opposed and helically extending seal ribs (19, 20) on the drive member (10) is arranged to sealingly cooperate with a second set of two diametrically opposed and helically extending seal ribs (21, 22) on the output spindle (12). The pitch of one of the ribs (19) on the drive member (10) is equal to the pitch of one of the ribs (21) on the output spindle (12), whereas the other of the ribs (20) on the drive member (10) has a pitch that is equal to the pitch of the other rib (22) on the output spindle (12) but different to the pitch of the first mentioned ribs (19, 21). Thereby, a sealing cooperation of the seal ribs (19-22) occurs just once per relative revolution between the drive member (10) and the output spindle (12).