Non-metallic gage for gap

The present invention concerns a non-metallic gage for detecting a gap which can not be seen from the out side such as a gap between a water pipe and fuel rods without damaging an objective material as to whether the gap is formed within a standard value or not. The gage is made of a synthetic resin, for example, polyacetal having such a hardness as not damaging the objective material and endurable to repeating flexure upon use. The gage comprises a short gage portion having a predetermined standard thickness and an flexible extended connection portion reduced in the thickness. Provision of the extended connection portion enables wide range flexure thereof such as ±60deg relative to insertion direction during insertion operation upon testing to solve a drawback in the prior art such as worry of breakage of the gage, thereby enabling to conduct inspection rapidly at high reliability. (N.H.)