The Pro-Built® 4X4 rear differential comes professionally assembled for easy drop-in replacement. The sealed assembly comes pre-filled with 30k-weight silicone oil with steel gears to withstand Velineon® brushless horsepower. Pro-Built differentials are direct replacements for VXL 4X4 models (see full list below). Includes 13-tooth pinion gear and 37-tooth ring gear. (Pro-Built differentials are not directly compatible with Titan®-powered 4X4 models)