OS #8, on a standard head. I usually run Duratrax Gold plugs in my old Hyper 8P and Outlaw 26, but I didn't want to catch hell for putting one in a Sirio. So far I have not had any problems with the 8's, but it seems like when it gets some use on them the tune gets off (or won't start). Fresh plugs run great though. I guess it's due to the higher grade of mill that makes it act that way. After about 1+ gallons, I'm on my second plug (not counting the breakin plug). As a side note, I'm burning Blue Thunder Race 20% in all my rides.