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