housing application_number US 96721978 A description in the figures 10 designates the machine housing which accommodates a cylinder bore 11 axially confined by a forward end wall 12 and a rear end wall 13 the forward end wall 12 is formed in one piece with the housing 10 whereas the rear end wall 13 is removable from the housing 10 but is kept in place by a lock ring 14 the rear end wall 13 is sealed off relative to the housing 10 by means of an o ring 15 the end walls 12 13 have cylindrical central openings 16 17 to receive a motor unit 18 therethrough the motor unit 18 comprises a pneumatic vane motor and a power transmission which parts are of a conventional design and are therefore not shown in detail at its forward end the motor unit 18 rotatively supports an output shaft 19 for connection of for instance a drill chuck exteriorily the motor unit 18 comprises two cylindrical parts 20 21 of different diameter the forward wider part 20 is guidingly received in the opening 16 of the forward end wall 12 and is sealed off by an o ring 22 the rear slimmer part 21 is guidingly received in the central opening 17 of the rear end wall 13 and is sealed off by an o ring 23 the rear part 21 of the motor unit 18 comprises a pressure air inlet passage 24 and an outlet passage 25 both communicating with the non disclosed vane motor at its discharge end the outlet passage 25 is provided with a noise muffler 26 of a porous material between a shoulder 27 which separates its two cylindrical.