All steel, gloss grey & chrome. Inside nub for 17" & 18" caps is 7". Inside nub for 20" cap is 7 1/2". 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 516-8010400 to achieve backspace. Doesn't fit Jeep Samurai. Dual bolt pattern wheels have 10 bolt holes. Please note our 20" Smoothie center DOES NOT have the ring around the cap like our 15-18" centers do. Image shown is 20". See Series 510 for picture of center detail on 17" & 18" centers.