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