The OEM chain has only 7,000 Tensile LBS. You could get a better aftermarket chain with 8,500 Tensile Lbs for the same price.
I personally do not like the OEM chain & think Yamaha have fitted a weak chain for the torque of our XT"s. With chains reported to break from 4000kms, mine only made 18000 kms & it was stuffed.
Go & buy a good Xring chain.
http://xt660.com/showpost.php?p=28949&postcount=19