Gauge for detecting diameter of external thread and detection device

The invention relates to a gauge for detecting the diameter of an external thread and a detection device.The gauge comprises a plurality of sleeves which are sequentially arranged in a sleeved mode layer by layer, the side wall of each sleeve is provided with a thread detection hole, and the adjacent sleeves can slide and stretch out and draw back mutually; therefore, the sleeve located on the inner layer has an extending position and a retracting position relative to the sleeve located on the outer layer, and at the extending position, the thread detection hole of the sleeve located on the inner layer is exposed out of the sleeve located on the outer layer. According to the gauge for detecting the diameter of the external thread and the detection device, the adjacent sleeves can slide and stretch out and draw back mutually, and at the stretching-out position, the thread detection hole of the sleeve located on the inner layer is exposed out of the sleeve located on the outer layer, so that the thread detectio