The differential spool replaces the standard differential to lock the axle and provide equal power to both wheels. For straight-line sand drags, rock climbing, or pulling, you can replace both the front and rear differentials with spools for total 4WD traction. On tight off-road tracks, install a spool in the rear diff case to allow the rear end of the truck to rotate more easily and slide through the corners. Corrosion-resistant finish and tough steel construction ensures lasting durability. (Recommended for off-road use only) Requires 5379X or 5379R ring gears.