Hydraulic lifters are designed to be self adjusting, they still have a running clearance but drop the volume of retained oil when a certain pressure is released, pressure is affect by contact with the cam lobe so the concept is they build up pressure which takes up slack/wear and then bleed off to give a running clearance. That's why they sometimes 'clatter' on start up when the engine has been left standing for a while but soon, hopefully, correct and all is well. Some engines are more prone to this than others, some never have the problem.....as an aside some historically had issues of 'pump up' when sludge blocked the hydraulic follower drain holes and clearances reduced and valves burnt out, regular oil changes helped reduce this.
