wherein the crankcase acts as a third working chamber by virtue of the pressure from the working medium acting on {HEAD ~ the underside} of {TAIL ~ the pistons}, 
wherein the crankcase acts as a third working chamber by virtue of the pressure from the working medium acting on { HEAD ~ the underside } of { TAIL ~ the pistons } ,
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 REL TAIL TAIL TAIL TAIL TAIL TAIL OTH