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