A tall valve cover is, by definition, an aftermarket replacement of the OEM valve cover. Tall valve covers are necessary to accommodate other aftermarket modifications or replacement parts, such as larger rocker arms or higher lifting cams.
There is a drawback to tall valve covers that should be taken into account before modifying newer engines in a manner that will require replacing the OEM valve cover with a taller version. Tall aftermarket valve covers are not custom designed for all optional extras. As such, they may not fit in the tight engine compartments of late model vehicles. OEM valve cover height clears accessories like air conditioning: a tall valve cover may not.
Stock valve covers for both small and big block Chevrolet engines are 2 1/4 inches in height. A tall valve cover for a small block Chevy is 3 1/2 inches high. A tall cover for a big block Chevy is 4 1/4 inches high.
You will notice that, though small and big block engines come with a short valve cover of the same height, the tall version is 3/4 inch higher for the big block as compared to the small block. This is because there is more room inside a small block OEM valve cover than there is inside a large block OEM valve cover. Consequently, some valve-train modifications -- roller rockers, for instance -- will fit inside a Chevy small block short valve cover. Installing roller rockers in a big block Chevy, on the other hand, does require replacing the short valve cover with a tall one.
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