wherein said front face portions and said rear face portions form substantially equally sized and configurated but oppositely directed curved face portions with bearing face radii around said first and second bearing face axes which are distanced in opposite directions from {HEAD ~ the vertical medial plane} through {TAIL ~ the respective piston shoe}, which are substantially equally distanced from the respective portion of said guide face and which are parallel to said first axis but distanced therefrom, and , 
wherein said front face portions and said rear face portions form substantially equally sized and configurated but oppositely directed curved face portions with bearing face radii around said first and second bearing face axes which are distanced in opposite directions from { HEAD ~ the vertical medial plane } through { TAIL ~ the respective piston shoe } , which are substantially equally distanced from the respective portion of said guide face and which are parallel to said first axis but distanced therefrom , and ,
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH