1級の通り側ゲージgrで合格したおねじが2級のgrで合格しない＜ゲージの公差とねじの公差＞ | オーエスジー

There is a difference in the second-class accuracy of the external thread with the reference effective diameter.

Class 1 threads start with a reference effective diameter.

If the workpiece enters during this time, this will be the result.

References: gauge Tolerances and Screw Tolerances

(FAQID:203)