portion 24 by virtue of a slight interference fit and rotatably mounts and centers the input shaft 15 within the housing 11 a demountable snap ring 26 which seats in a suitable circumferential groove 27 disposed in the input shaft 15 positions the input shaft 15 axially relative to the bearing 25 and unidirectionally maintains this relative position the circular bearing support 22 also defines a plurality of longitudinally disposed apertures 28 which are disposed in the flat radially extending region of the circular bearing support 22 and which provide communication between its two faces the circular bearing support 22 also defines a circumferential grooves 29 within which is seated a resilient o ring seal 30 which may be fabricated of rubber neoprene or similar material an annular spacer 31 is frictionally retained over the portion of the circular bearing support 22 adjacent the o ring seal 30 by means of both a slight interference fit and the additional frictional retention afforded by the o ring seal 30 the o ring seal 30 also provides a substantially air tight seal between the circular bearing support 22 and the annular spacer 31 the cylindrical housing 11 defines a plurality of radially disposed apertures 32 which are spaced circumferentially about its periphery the cylindrical housing 11 further generally defines a wide circumferential re entrant groove 33 an annulus 34 having a plurality of radially oriented exhaust apertures 35 disposed about its periphery seats over the groove 33 in suitable circumferential channels 36 and defines a chamber 37 a passage for exhaust air from the housing of the air motor 12 is thus defined and extends from the plurality of apertures 28.