Our company purchased a set of Grade B Gage Blocks years ago that only receive moderate use. We send them out to our calibration vendor every two years for recalibration. When we purchased the blocks, we understood the tolerance to be ±50 millionths of an inch. Now our vendor says that they should be calibrated to Class 3 tolerances of +8/-4 millionths of an inch. I briefly searched the web and could not find information on Class B specifications or why they would need to be calibrated to Class 3 tolerances. Does anyone here know the history of these classes and can explain it to me? If we calibrate to Class 3, we will have 3 of our blocks fall outside the tolerance by 2 or 3 millionths of an inch.