BBC Fabricated Aluminum Valve Covers, Machined for PN 22635 -12AN Positive Sealing Fitting (included) Valve covers are 3 " tall for symmetrical-port Bowtie cylinder heads and Stock BBC heads, clears Jesel, T&D Rocker Shaft Assembly. One-Piece 3/8" Billet Rail is CNC machined from 6061-T6 Aluminum to ensure a precision flat surface for superior sealing. Steel inserts pressed into mounting holes in rail to eliminate galling after removal and installation. Rail is CNC after welding to ensure airtight sealing.