So just how do these lifters work? The concept is a self adjusting thickness lifter that keeps the lifter in contact with the cam shaft and top of the valve at all times.
Intentionally these HLAs (Hydraulic lash adjusters) are designed to let oil flow in between the inner and outer plunger assembly in only one direction. The oil pump pushes oil past the check ball in the inner plunger. This oil pressure (combined with the plunger spring pressure) pushes the outer plunger away from the inner plunger, which makes the HLA grow taller (the outer plunger pushing down against the valve, and the inner plunger pushing the lifter body towards the cam shaft.).
Then BAM!!! All of a sudden the cam shaft lobe is pushing down with great force on the Lifter Body, which transfers that force to the inner plunger. This reverse pressure tries to force the oil between the inner and outer plunger back out the hole in the center of the inner plunger. OH wait! Theres a check ball that now gets slammed shut from the pressure. The oil has nowhere to go. The two plungers fit together so closely that the oil cant even squeeze between their walls The two plungers are immediately in Hydraulic lock, and the cam shaft forces the valve open pushing all the cam force through the oil trapped between the plungers. Works like a charm.
Theres just one drawback: the oil never gets out. Ok maybe a very small amount escapes around the walls, or past the check valve, but its still trapped inside the built in reservoir of oil that the top of the HLA has sitting there ready for getting stuffed back in between the plungers. And the HLA reservoir top is designed just like plungers, one little hole flowing oil in (under oil pressure) and no holes out. So this oil is essentially stagnant inside the HLA.
Stagnant oil wouldnt be bad, except for when it breaks down and thickens up. Then the oil pump cant force enough oil in through the three small holes and past the check ball to ensure that the chamber in between the plungers stays pressurized. Not enough oil pressure means that the chamber between the plungers shrinks by enough to allow a small space between the valve and lifter or cam and lifter. This small space allows the metal to impact metal making our favorite HLA Tick Tick Tick noise.