Mechanical solid-rod inside micrometers and sets have interchangeable rods that are used to measure the inside dimension of holes, tubes, pipes, cylinders, rings and other parts. These instruments have interchangeable solid rods of different lengths that fit into the body of the micrometer. The ratchet thimble on the body is used to extend the rod until it makes contact with the sides of the bore or feature. When the rod makes contact, the micrometer is locked into place and extracted. The user adds the length of the rod being used to the main and vernier scale reading to determine the total measurement.