Lifters have a convex bottom and cam lobes have a tapered lobe from front to back. When the tapered lobe goes flat across there is nothing to spin the lifter properly. You might get a little rotation from the lobe being offset to the lifter but the taper is necessary to working properly. At this point the wear accelerates and you end up with lifters with concave bottoms like the ones pictured. I say that the cam already had went past the wear point and the lobe had very little or no taper. The one flat tappet lifter brand you hardly ever hear of going flat is the Crower but even these will fail when installed with a worn cam. You really should still see some Parkerizing along the lobe profile which indicates that there is enough of a taper to possibly run the cam again. Of the 11 flat tappet cams I've pulled out of engines just this year prior to the rebuild, I wouldn't have put any of them back in.
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Mick Batson
1967 original owner Tyro Blue/black top 4-speed HO GTO with all the original parts stored safely away -- 1965 2+2 survivor AC auto -- 1965 Catalina Safari Wagon in progress.
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