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.