Lifters, in most OHV engines, are hydraulic (there are "solid" lifters - they're just pieces of metal, and require frequent adjustments.)
Hydraulic lifters have an oil cavity, a plunger, and a valve in them (usually, the plunger going over the hole in the body does the valving.)
In old solid-lifter setups, it was necessary to adjust "valve lash," or operating clearance for the lifter. This was done betwen the valve tip and the rocker arm tip, and could range anywhere from .005" to .015". Instructions were different for each engine - many were adjusted "cold," but some were adjusted "hot", some while running, ...
Anyhow, a hydraulic lifter displaces the operating clearance - rather than being between the rocker and the valve tip (which just translated to a slight clearance everywhere...) it is meant to be entirely within the lifter. The plunger has somewhere around .030"-.035" of travel, which is supposed to allow for lash adjustment at all temperatures (lash reduces as materials expand at operating temperaure...)
The valve lifter is "pumped up" by engine oil pressure, and this has the side effect of pumping oil up to lubricate the top end (via the pushrod -which is why most of them are hollow.)
Now, a valve lifter can fail in a small number of ways:
"Go Flat" - a lifter is made with a very slight "crown" on the foot - it complements a sidewise taper on the cam lobe, and is meant to rotate the lifter in normal operation. A lifter that's "gone flat" has lost that crown or has somehow been restricted in rotation, which rapidly results in losing that crown and the cam lobe WILL wear a groove (about a cam lobe wide...) in the lifter foot. Oops. Can also be caused by crud, grit, or a burr in the lifter bore, preventing the lifter from rotating. (This does not apply to roller lifters - just "flat" tappets and "mushrooms.")
"Go Solid" - a lifter that has "gone solid" has the plunger held at full uptravel - usually, this is caused by sludge or contamination. Pennzoil has cause a great number of "solid" lifters, in my experience - so will infrequent oil changes... This is a reference to old "solid" lifters - they didn't have moving parts. (Does not apply to solid tappets)
"Collapse" - a lifter that has "collapsed" is suddenly unable to retain oil pressure needed to self-adjust the lash out of its operating train. Simply put, the plunger goes to the bottom of the travel and stays there - this is usually caused by wear of the plunger in the body/bore. A slight leakdown is normal (that's how oil gets pumped up to the top end,) but excessive leakdown will make it sound like an old "solid-tappet" engine. Or maybe a Diesel. (Does not apply to solid tappets)
A quick check - the typical "flat" tappet doesn't have a flat foot - there's a very slight crown there, it corresponds to a circle that's about 30" in radius. Therefore, you can check the "crown" simply by putting the lifter foot on a flat surface (I have a glass that I use - not a surface plate, but it works) or putting two lifters "foot-to-foot" and trying to rock them around. You should be able to rock the lifters - if you can't, they're probably made truly flat and should be exchanged.
Make sense now?
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