[PeteVS]

Although without the proper tools, you won't be able to accurately measure the inside diameter of the drums, you should be able to clamp two pieces of scrap side to side with the ends touching the inside of the "small" drum. Once you have them clamped, take them over to the "larger" drum and you'll be able to see what the difference is. I'm going through a similar situation but I'm using new drums and either new shoes or relined shoes. I don't know which. I have a 12" Starrett steel rule (slightly flexible) and it has to be flexed slightly to fit into both drums making me think that they might be a wee bit undersized.