When new, they -usually- can be compressed & relieved. When primed, it´s IMPOSSIBLE to compress any of them even a millessime of inch.
From prehistory (early forties) to last year, lifters ALWAYS primed for themselves just dropping them in da hole, timing accurately the pushrods and starting the mill: circulating oil primed the lifters and the horrorous noise from down there goes gradually loosening clicketies to the sweet rumour it has to be.
I'd say last year because my last lifters set did HAVE to be primed very very carefully. Sure da hell you have up there in the USA lots of Buick lovers aimed to explain you how to it. My English is mmm... cathastrophic.
Finally, I repeat: When primed, they have ZERO tolerance to compression effort. If they allow a mere 1/1000" plunging under hand-and-rod action, you'll have great heat & performance troubles in all the engine.
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