two pistons connected via {TAIL ~ connecting rods} to {HEAD ~ a single eccentric crank pin} of a crankshaft, 
two pistons connected via { TAIL ~ connecting rods } to { HEAD ~ a single eccentric crank pin } of a crankshaft ,
OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH