Abstract: A tool for incorporation in a drill string has an outer body (11, 12, 13) and a spool (17, 18, 19) rotationally and slidably mounted within the body. A cam groove (20) is formed on camming component (17) of the spool and interacts with pins (21) to cause uni-directional spool rotation through a number of pre-set angular positions as the spool is reciprocated. The valving component (18) of the spool is closed internally by wall (26) and has orifices (25) which come into and out of registration with body openings (29) as the spool is moved through its pre-set positions. When in registration, flow into the tool will leave the tool radially outwardly through openings (29); but when out of registration, the flow will be into chamber (28) of body central component (12), and back into the valving component (18) downstream of wall (26) through apertures (27).