Hydraulic lifters do not simply fill once, and stay that way. The size of the oil galleries that connect all the lifter bores from front to rear and the volume of oil from the pump needed to keep them full tells me that lifters use a lot of oil even when they're operating normally. At a cruise RPM of 2000, each lifter is cycling its valve open and closed 1,000 times a minute, or almost 17 times a second. While the cam lobe is lifting the valve, the oil trapped in the lifter body under the plunger acts like a solid lifter, since liquids can't be compressed. In the process of doing the lifting, a tiny volume of oil is lost due to the clearances built into the lifter plunger and the outer body. As the valve returns to its seat and the tension on the lifter is at a minimum, the pressure of the oil pump refills whatever oil has been lost during each lift of the valve. That's why getting the right preload on the lifter is so important- - - -When it is at its lowest point of travel, the bits and pieces of the lifter must align correctly for the refill to happen reliably.
Trying to adjust a lifter successfully with the engine running has two possibilities- - - - -slim and none! It's a terribly inaccurate adjustment method at the very best.
Jerry
