OK, first understand that I do not know jack about internal engine parts. However, an Internet search of FAQs on cam websites and posts on old vehicle discussion boards provides a consensus opinion that is succinctly presented in this post on
chevytalk.org :
"For hydraulic flat tappet/mechanical flat tappet:
New lifter on a old cam is OK (as long as the lobes are still OK) - you just have to go through the 20 minute/1500-2000 rpm break-in procedure again. This wears in the lifter to the cam.
Old lifter with new cam is NOT OK - the new cam will accelerate the wear on the old lifters, causing them to fail.
Old lifter/old cam is OK, as long as they were worn in together and the lifters go back in exactly the same place on the cam.
"While perhaps not desirable, if the old cams lobes are in good shape, new lifters may be installed as long as the normal lifter/cam break-in procedure is used.
Now, let the free-for-all discussion continue.
