PREC. BILLET Bad Axe Billet Derby Cover - Chrome BAX-230-BG0-CHR Derby Covers are not all created equal. Some designs appear flat or with little definition, but not ours. The Darkside HD Derby Covers by Precision Billet have a look that pops. The 3D design adds definition to the derby cover but does not interfere with the moving parts around it or make it too thick. Precision CNC milled from 6061 T6 aircraft aluminum.