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.