it's a 10 mm bolt; i used a box end wrench; it gives you finer adjustments than an open end.
Also (and this will probably jinx you), DON'T DROP THE WRENCH! I did, and it turned a 10 min. job into a 30 min. one with my arm buried up to the shoulder trying to fish the wrench off the top of something i couldn't get to from underneath.
Wish i had one of those little magnetic or claw-type grabber thingies.
Peace
Jed
