We can replace the hydraulic lifters of tappets. Hydraulic lifters & tappets rely on oil to enter which controls the valve clearance. You may notice that on initial engine startup the valve train is noisy until oil pressure is up and the lifters are actuating correctly. You generally see either a cylindrical design or a bullet-style lifter with a ball like top. These are sometime referred to as ball stud adjusters and the ball fits inside the rocker arm.
