19 rotatably mounting actuator arm 3 to pivot shaft 15 pivot shaft 15 is fixedly secured to the chassis 1 by screws 21 ball bearings 23 are mounted on shaft 15 to maintain the desired spacing of actuator arm 3 with respect to chassis 1 and thus maintain the required spacing relationship between the heads 9 and the surface of the disc 2 referring now to fig3 and 4 the actuator arm 3 including collar 17 and head support plate 5 is cast from a suitable light weight non magnetic material such as magnesium the casting is a single piece having a slot 24 cut into the arm 3 forming collar pieces 17 39 and arm members 25 insert 27 is coated with an insulating material and bonded into place positioned between arm members 25 plates 28 and head support plate 5 define an open cavity 38 into which coil 35 is inserted and bonded in position fillets 37 provide additional stiffness for arm 3 and insure accurate placement of coil 35 in cavity 38 head support plate 5 cast integral with actuator arm 3 comprises support structure 34 and support arms 36 to which the head flexures 7 are secured with screws as shown in fig1 and 2 coil 35 is inserted through head support plate 5 into cavity 38 and positioned between fillets 37 referring now also to fig5 the actuator arm 3 is driven by coil 35 disposed within cavity 38 coil 35 is wound in a generally rectangular form having an open center conforming to the size and shape of cavity 38 the cavity 38 and coil 35 are located at the center of percussion of actuator arm 3 to minimize forces on the pivot shaft 15.