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