Same as the complete BDL primary chain drive kit (like 518724 and 518629), 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. 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.