Now how ya gonna change the retainer without dropping the valve into the cylinder ?????? One method is an air hose screwed into the spark plug holewith the rocker arms off for that cylinder air preasure will hold the valves closed for ya....
another method if'n ya don't have a compressor or the air hold tool is get some clothes line rope fill the cylinder with it thru the spark plug hole and gently turn the motor over ( by hand) untill the rope compresses and holds the valves closed...... Be sure and leave some rope hangin out so you can remove it