You won't break this cover, we guarantee it. If you do, we'll replace it. Made of 3/8 inch cast steel, painted black inside and out, this heavy duty cover is designed to protect your differential and all its working parts for as long as you own your vehicle. This cover even has reinforced protection for the fill plug. All mounting hardware and instructions are included.. This heavy duty differential cover is made of 3/8 inch cast steel and is designed to protect your AMC 20 differential and all of its interior parts.