Drums typically have cast on the outside of the drum printed "MAX. DIA." which means maximum diameter. This is to let you know up to what point the drum can still be used. Because as the brakes apply, brake shoes make contact to the inside of the brake drum and waste the inner lining of the drum, and as this happens the inside gets bigger and bigger. To check the inside diameter of a brake drum you will need a Fraser Drum Gauge and know the maximum diameter of the drum you will be checking, in my case it is 16.620". Get your Fraser Drum Gauge and slide the bar to the desired length, the bar is set in inches and increments of 1/4" (or 0.250 of an inch), so for example from 15 to 16 on the slide bar, one line between 15 and 16, is 0.250, the next line would be 0.500, and the following would be 0.750 and after that it would be 16. There are only 3 lines, its like a dollar if you add up your quarters you go to the next dollar value. So i set my Fraser Gauge to 16.500, because my max diameter is 16.620, if i were to set the gauge to 16.750 that would be over the max diameter and i wouldn't get a proper reading, and the gauge probably wouldn't event fit?. Ok now that there is a understanding of the slide bar, now place the Fraser Gauge inside the drum and check the drum one inch down from the edge, once that has been accomplished, move the gauge side to side, your gauge will start to make the arrow move up and down on the numbers displayed on the gauge, try to get a steady number in between. In my case the arrow on the gauge pointed at "10" which was "0.010" if it landed on "30" it would be 0.030, and so forth. So now get your numbers and add them up to get you readings. In my case 16.500 plus 0.010 equals 16.510, so this drum is well under the max diameter of 16.620 and still can be used.