All 2st Vertex or Wossner Piston??

The tool what ever it was called (he brought many), took the inside diameter of the cylinder (66.34).
So if we measured the above measurement, and specs for my bike say 66.40, then my question remains, will this be ok?

http://www.motorcycle-usa.com/22919/Buyers-Guide-Specifications/2007-Husqvarna-WR250.aspx

Oh, sorry I must have misunderstood, I thought you were saying your piston measured 66.4 and your cylinder measured 66.34. If your cylinder is round and measures 66.34 it will be fine as long as your piston measures about .06mm smaller than the bore for a cast piston or about .09 if it's a forged piston. The replacement piston will have the clearance spec on the box or on a spec sheet. If it falls within the spec you are good to go.