Same as the complete BDL primary chain drive kits (MCS 518708 and 518628), but this kit includes the clutch assembly only. The clutch shell includes a starter ring gear for electric start options and contains the BDL competitor clutch for positive shifts and no clutch drag. This chain drive kit will fit inside the stock primary housing. You can re-use you existing primary chain, chain adjuster and starter gear. Note: Compatible with a 25 tooth motor sprocket only (as 515121 BDL compensating sprocket). This must be purchased separately.