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