wherein said {TAIL ~ slide faces} and {HEAD ~ piston shoes} have a pair of extensions at opposite ends of said medial portions in the direction of the movements of said piston shoes, 
wherein said { TAIL ~ slide faces } and { HEAD ~ piston shoes } have a pair of extensions at opposite ends of said medial portions in the direction of the movements of said piston shoes ,
OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH