Why do you want/need to know the cylinder wall thickness? If a B23 is just a bored out B21 from the factory (and I personally don't know this for sure) then it seems safe to assume that cylinder wall thickness will be adequate. If your worried the machine shop will bore on a skew instead of along the true cylindrical centerline, consider another machine shop. If your concerned that inadequate wall thickness will remain try to ascertain what the starting thickness is and subtract the amount of the overbore, remembering to take 1/2 the diff if your talking diameter. Hope this helps at least a little.