This is an optional Schumacher CAT L1 EVO Complete Aluminum Differential, intended for use with the CAT L1 EVO 1/10 4WD Off-Road Buggy Kit. The aluminum case gives super precision and strength and has a larger oil volume for more consistency during a run. Main large shim also thicker for more reliability. Build a complete diff with all parts necessary included. SCHU7979 is the conversion if you already have the diff parts.