Stainless Steel Unlike other hex-head designed oil restrictors, Stef's restrictors install with a 5/16'' socket head and ''hide'' inside the block so they won't interfere with mid-plates, block savers or flywheels. They feature 1/4'' NPT threads, are reusable and can be installed on early or late-style blocks.Fits SB/BB-Chevy (including bowtie). 2/pkg.