Replacing a lazy lifter

The piston in the lifter sits somewhere between being bottomed out in the lifter housing and all the way to the top against the retainer that keeps it all together. The exact position is determined by all the tolerances in the engine and in the case of adjustable rockers where you adjust them to. The flow of oil through the lifter galleys will keep them pumped up to the point where there is zero lash, it won't pump them all the way up.

If you soak new ones in oil before installing them the spring inside has the piston against the retainer and they fill up with oil. When you first start the engine the piston is already at the top of its stroke, not where it needs to be to keep the lash at zero and it will hold the valves off the seat.

It takes longer for the piston to bleed back down to where it needs to be than to fill up to move the piston to where it needs to be. With the valve off the seat the potential for damage exists. The clatter at start up while they fill is annpoying but not dangerious for the engine. This is the reason you don't soak them in oil.