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