A groove 26B is formed on the outer peripheral surface of the measuring element 26, and the forward end of an engaging plate 28 pressed in from the bottom surface 10C of the head 10 is engaged with this groove 26B, {HEAD ~ whereby a forward limit stopper} and a rearward limit stopper for the measuring element 26 are constituted by the engaging plate 28 and {TAIL ~ the groove} 26B 
are constituted by
A groove 26B is formed on the outer peripheral surface of the measuring element 26 , and the forward end of an engaging plate 28 pressed in from the bottom surface 10C of the head 10 is engaged with this groove 26B , { HEAD ~ whereby a forward limit stopper } and a rearward limit stopper for the measuring element 26 are constituted by the engaging plate 28 and { TAIL ~ the groove } 26B
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