side 150 includes the alignment holes and a circular guide wall 158 which extends from the port plate cylinder side 150 to circumferentially engage the cylinder wall 118 the circular guide wall 158 has a diameter slightly larger than the outer diameter of the cylinder wall 118 preferably a plurality of fingers not shown extend axially in the annular space defined by guide wall 158 guide the valve plate 35 formed on the cylinder closed end 116 within the guide wall 158 to align the apertures 124 126 with the port plate ports 142 148 the port plate head side 152 includes a curb 162 surrounding each of the ports 142 148 and an outer groove 164 surrounding the curbs 162 two center grooves 168 170 connect the outer groove 164 between the ports 142 148 and respective curbs 162 a center wall 156 is formed between the center grooves 168 170 and curbs 162 and defines a lengthwise edge of each center groove 168 170 a registration opening 172 formed in each curb 162 receives a finger 174 extending from a port plate gasket 176 received in the grooves 164 166 the port plate gasket 176 is received in the outer groove 164 surrounding the ports 142 148 and has a center member 180 which is received in one of the center groove halves 168 170.