Bit and Shell Wear Gauge 

As good practice, the diameter of surface set bits and reaming shells should be measured. This will ensure that the hole size is maintained. All holes should be collared with a full gauge reaming shell. On deeper holes, two or more shells should be alternated in the hole to maintain the hole size. Reaming shells should be gauged every trip out of the hole. The practice of gauging bits is more common with surface set bits. Impregnated bits typically maintain their gauge until totally consumed.