.002-.003" should do. Best way to make sure reaming it perfectly in the middle you need a bridgepprt or similar mill and take a drill bit or gauge pin that fits very snug in the hole of the arm and put it in the chuck then move the handle down and align the bore gauge or drill bit in the arm and snug up the vise the arm is in and make sure the table of the miill doesn't move at all. Drill it out close to the size u need then find a reamer of the correct size and ream it
