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The problem is as much with the lifters as with the cams.
The OEM lifters have a very hard skin which is not very
thick on the surface that rides on the cam. If you find an
OEM lifter that is anything but dead flat and shiny, you can
pretty much count on the cam being damaged. Once the
skin wears through, the edge is hard and sharp and is almost
like a cutting tool on the cam. The hardest area to keep
lubricated is the cam/lifter interface because it is mostly a
line contact with very high unit pressure.
A check on the cam is to put a ruler longitudinally
on the peak of the lobe in question. If it is at all
rounded lengthwise (doesn't touch the ruler all the
way across) it has started to go. I think this problem
was worst in the mid 70s but have also seen it in
B18s. Do NOT under any circumstances try to
"save" a damaged lifter. It will only destroy your
cam.
Also cams may go in anywhere from a couple minutes to
several thousand miles depending on condition of lifters
and driving circumstances.
Re: valves too tight -
They may be OK after being run "tight" too long but
chances are good that if you tear it down, you'll find
pits or cracks in the seating area of exhaust valves
run without enough clearance. May run OK for quite
a while. Once you get a crack, though, it is a tiny
flame passage during the combustion stroke, so it
will indeed get bigger and worse and continue
to cause the valve to run hotter than it otherwise
would.
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