the pistons forming first and second {HEAD ~ working chambers} in {TAIL ~ two cylinders}, arranged 180° apart so that when one piston is at top deadcenter the other is at bottom deadcenter, 
the pistons forming first and second { HEAD ~ working chambers } in { TAIL ~ two cylinders } , arranged 180 ° apart so that when one piston is at top deadcenter the other is at bottom deadcenter ,
OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL 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