For high pressure gas cylinders and tanks, such as scuba diving tank, firefighter oxygen cylinder, or medical oxygen tank, visual inspection is very important for the safety of users and the long lifetime of the cylinders. The neck thread is one part needs to be visually inspected. Here are several tools used to exam the internal thread surface for cracks, metal loss, dents, distortion, corrosion, foreign objects, and thread damages.
Current tools range from a very simple one, like a dental mirror and a small flashlight, to elegantly designed ones. One drawback is that it is not convenient to use with a traditional eyepiece; another drawback is that the magnification is low, from 2x to 4x. More innovation is needed. Below is a 1998 article by Luxfer on the topic.