Have you ever had problems adjusting rocker arms on your car Kiss those problems goodbye with these rocker arm shim kits from Ford Racing. They are to be used with all bolt - down style roller rocker arms to obtain correct lifter preload. These kits include 16 thin and 16 thick shims, which can be used to reduce lifter preload by .030 in. (thin shim), .060 in. (thick shim), or .090 in. (thin plus thick shim).