The normal clearance for lifter parts is about 2/10,000" (0.0002") That's a strand of hair cut into 10 equal slices. A little wear can let the plunger get cocked in the bore and get into a bind, plus the leakdown rate increases. Automatic transmission valve body parts can do the same thing, with no measureable wear. Ford AOD's are chronic for that problem, with their aluminum valves running in a steel valve body.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!