a lot of time went into the fitment of the o-ring placement and depth ,also being able to have a cover that the top 3 bolt pockets that hold the cover down look like the lower 4...these are the ONLY covers that have this done to them, and the ONLY billet covers for these heads that are available for sale, And are only a few hundred dollars more than a high quality set of welded sheetmetal covers... The machine shop that is producing these for me can do engraving as well...these will be the last set of covers anyone ever will need...I have ARP bolt and stud kits for these covers available also.....