The plunger only moves toward the chain. To disassemble it the plunger had to move toward the chain and then out. Put the chain completely in place, put the body of the tensioner with gasket onto the head and snug it in place, lubricate the plunger and install it from the outside of the head into the body of the tensioner, put the spring into the plunger, put the seal on the big nut. STOP and go eat Wheaties and spinach, reinstall the big nut whilst overcoming the spring tension, it's not easy. Done.

__________________
"Rudeness is a weak man's imitation of strength" - Eric Hoffer