Both modern mechanical and hydraulic lifters are made the same way- - - -the difference is the plunger on a hydraulic that can slowly displace the oil inside and adjust its height to be sure the valve closes. The mechanical lifter made on the same body has an upper section that cannot retract. You can get a valve fully closed and put firm pressure on the rocker arm with a pry bar for a few minutes and a hydraulic lifter will slowly bleed down and increase the gap at the valve stem to almost 1/8", then it will pump up again as soon as the engine is started. A mechanical lifter will not bleed down when the same procedure is tried.

Both types of lifter will have a snap ring in a groove near the top that holds the pieces of the lifter together. The inner piece will retract under firm, steady load on a hydraulic lifter.
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!