referring to the drawing , fig1 illustrates a nut 10 mounted on a threaded stem 12 . power tool means 14 is adapted to be operatively connected to nut 10 to rotate it to a predetermined position along stem 12 in order to locate a member 16 with respect to a mechanism not shown . power tool 14 includes a movable fixture means 18 supporting an air motor 20 for raising and lowering the air motor with respect to stem 12 . air motor 20 is rotationally connected to a rotor 22 carrying a socket wrench 24 adapted to engage nut 10 . air motor 20 has a housing 26 enclosing a power chamber 28 which includes a cylindrical wall 30 . wall 30 is formed about an axis illustrated at &# 34 ; a &# 34 ;. rotor 22 is connected to a rotor body 32 disposed in the power chamber to rotate therein about an axis indicated at &# 34 ; b &# 34 ;, parallel to and spaced from axis &# 34 ; a &# 34 ;. body 32 has a series of annularly spaced slots 34 formed at regular angular intervals about axis &# 34 ; b &# 34 ;. a plurality of vanes 36 , each disposed in one of the slots 34 , are mounted in the chamber so as to be radially movable in their respective slots as body 32 is being rotated . power chamber 28 has an inlet 38 and an outlet 40 . referring to fig1 inlet 38 is connected by conduit means 42 to a source of fluid under pressure 44 . the connection is such that as pressurized air passes through power chamber 28 , the air imparts a rotational motion to the rotor body and the vanes before being exhausted through outlet 40 . preferably body 32 is connected by gear reduction means 46 to rotor 22 so it rotates one rotation for each two hundred rotations of the rotor body . an electro - magnetic sensing device 48 is mounted on housing 26 adjacent the path of rotation of the vanes . the vanes are of a conventional plastic non - magnetic material , however , body 32 is of magnetic material so that as each vane passes electro - magnetic device 48 , each vane slot causes device 48 to actuate a counter means 50 through a connection 52 . counter means 50 is connected by means 54 , 56 , 58 , and 60 to a source of electrical energy 62 . a solenoid valve 64 is connected to conduit 42 between source 44 and the counter means in such a manner that when the counter means reaches a predetermined value , it actuates valve 64 to block fluid flow from source 44 to conduit 42 thereby terminating rotation of rotor body 32 . the angular position of wrench 24 can be terminated such that the final rotated position of nut 10 on stem 12 can be positioned to within plus or minus 5 ° of a particular rotational position on stem 12 because of the responsiveness of the counter means to the passage of the vanes in the power chamber . for example , with five vanes in the housing , and gear reduction means 46 having a 200 - 1 reduction ratio , the counter means will be actuated one thousand times for each rotation of nut 10 . when the counter means achieves a particular value , it actuates valve 64 to thereby terminate rotation of rotor 22 and nut 10 with a greater precision than can be achieved with conventional means . in practice , a linear variable displacement transducer 66 is connected to the counter means . transducer 66 has a spring biased member 68 engaged with member 16 such that until nut 10 has displaced member 16 to a predetermined initial position , counter means 50 is inoperative . the transducer then actuates the counter so that the nut is only rotated through a predetermined angular displacement to its final rotated position on the stem . other means can be employed in place of transducer 66 for initially rendering counter means 50 inoperative until nut 10 has been located to an initial position . for example , the air motor can employ a transducer sensitive to the initial torque applied to the nut . the initial torque may be about five foot pounds as the nut is being initially rotated until it contacts member 16 . upon making such contact , the torque necessary to turn the nut increases because of the resistance of member 16 . the transducer is arranged so that it is sensitive to an increase in torque , for example , to eight foot pounds at which time it would signal the counter to commence its vanecounting function . mechanical calipers can also be employed for actuating the counter when the nut has reached a predetermined position on the stem . still other devices can be used for signalling the counter to respond to the vanes passing electro - magnetic device 48 . it is to be understood that although the embodiment of the invention is illustrated with respect to locating a nut on a stem , the invention can also be employed in other applications in which an air motor is used to precisely position an object in rotated position with respect to another object .