I bought my z start used and it has 9 tungston balls. As an auto clutch it works very well. The only real advantage of the core is the core has the normal cluch over ride. I would not hesitate to by the z start and do not think the core is worth the extra money. My core came on the bike when I bought it used so I did not need to shell out the dollars for the clutch. The core is a very good unit just pricey.