yep it is 15mm, but get a boxed wrench w/ a long handle from Sears or you will be stripping it. notice how grease monkeys always tighten them something insane? that's not because they are doing it on purpose: left to their own devices they'd leave it out altogether. it's just because you always get to it from a bad angle. with a nice wrench you will be surprised you ever had trouble w/ it before.
