After teaching lifter principles for 40+ years, I believe I can converse about them with a bit of understanding. Lifters are in a constant state of leakdown, otherwise they would pump up and force the valves open, killing the compression. When an engine stops with a lifter on the high point of a cam lobe, the lifter slowly bleeds down to its fully-collapsed position due to valve spring pressure. Within a few cycles of the cam lobe after startup, it pumps itself up to eliminate any clearance, then it expands or contracts during running to compensate for thermal changes in pushrod length. There must be a slight, but measureable flow of oil through a lifter for it to function properly. I used a weighted tester and a stopwatch for many years to test lifters for acceptable leakdown rates, including stovebolt lifters from PG engines. The adjustment procedure in the service manual sets the lifter in a partially-compressed position to align its internal oil passages, and also allow for expansion or contraction of the lifter's overall height without bottoming out or expanding fully against the retainer clip that keeps it together. The total available travel distance of the plunger is just under 1/4" on most lifters, and the normal running position is slightly less than 1/8" fron the topped-out position. There must be enough oil pressure to replenish any lost oil while the lifter is on the base circle of the cam between valve openings. Under thrust from the lobe for the fraction of a second while the valve is opening and closing, the lifter acts like a solid because the column of oil trapped between the lifter base and the plunger will not compress.
Jerry
