Brent: Speed220mph is right on, Motorhead and CJ428CJ have given a good description of the lifters themselves, the hole in the side of the body takes
oil volume and pressure as it travels up and down and passes it through the orfice hole that the pushrod sits on, through the pushrod and to the pivot at the rocker arm adjuster top side. With the exception of 'Shell Type" the lifter bodies are often times the same with just a seat for the pushrod held in with a clip, and yes there may be a little movement in the seat itself. All of the internal adjusting and bleed down mechanism is added by the manufacturer IF they are to be Hydraulics.
Rick