Do these sleeves or metal spacers extend out the bottom of the valve cover? If I compare this with the Fel-Pro gaskets using sleeves or spacers, they were not as thick as the gasket itself. So with the bolts tight the amount of compression of the gasket was limited. Without the spacers could crush the gasket.
But would stand to reason if all the valve cover bolts were loose, gasket wouldn't have any compression whatsoever. Another problem using plastic, is typically the rate of thermal expansive with temperature is greater than said cast aluminum.
Did read some post where oil leaks were noted, in particular on the inner part of that U. Loose bolts could be the cause, RTV would definitely help.
Didn't go that far to even remove a bolt, just that they were all loose. See what happens. But still wonder how the new valve cover differs from the old one.
Positive crankcase ventilation was a welcomed change when they came out in either the late 50's or early 60's. When a breather pipe was used, crankcase would have a vacuum that sucked up road dust when you took your foot off the gas. Would help if you shifted into neutral when coming to a stop. When overhauling an engine, you literally needed a shovel to get rid of the sludge in an engine.
Was fed back to the base of the carburetor so the carburetor would stay clean, but this all changed when manifold fuel injection was used. This crap from the crankcase would pile up on the face of the injectors and cause spray problems. So by fixing one problem, created another. All deals with having carbon in our fuel, very poor heating value with a host of problems.
Carbon build up always has been a major problem, not only this, but outputs CO2 and CO that is a killer. We tried to change this in the early 70's, but congress said, NO! Still saying NO!