All steel, gloss black. Inside nub for 14"-16", 20" & 22" caps is 7 1/2". Inside nub for 17" & 18" caps is 7". C52 cap included with all wheel sizes and is the only cap that fits 20". Lug nuts not included. A reverse rim is used for 510-8010400 to achieve backspace. Doesn't fit Jeep Samurai. Dual bolt pattern wheels have 10 bolt holes. Please note our 20" & 22" Smoothie centers DO NOT have the ring around the cap like our 15"-18" centers do. See Series 513 for picture of 20" & 22" center detail.