Abstract: Among the many challenges in the manufacture of oil seal rings of elastomeric material is to create a seal whose sealing surface is within certain critical dimensions, usually in terms of a nominal diameter plus or minus a tolerance. There have been several methods available to test oil seals to determine the actual diamter of the sealing surface. These require considerable experience and training. Repeatability can be adversely affected by a number of factors. The subject invention includes an apparatus which combines a light box with a controllably expansible gauge band. The expansion of the band is monitored by an electronic position indicator. In operation, the seal to be tested is placed over the properly selected gauge band on the light box. The user adjusts the vernier to expand the band until a "light seal" is obtained, i.e., when any space between the smooth outer diameter of the band and the lip of the the seal is just closed. At that point, a reading of the position detector is taken.