fixed jaw 2 is formed with a perforation 21 in which a thread rod 22 is disposed a spring 23 is fitted with one end of the thread rod 22 the other end of the spring 23 is connected with the fixed jaw 2 the other side of the fixed jaw 2 is sequentially pivotally connected with the gear 4 engaging block 5 and the grip 7 an upper cover 24 covers the fixed jaw 2 to enclose the above components between the upper cover 24 and the fixed jaw 2 a lower side of the movable jaw 3 is disposed with a rack 31 one section of which meshes with the thread rod 22 the other section of the rack 31 meshes with the gear 4 whereby by means of rotating the thread rod 22 the distance between the fixed jaw 2 and the movable jaw 3 is changed to fit different nuts 1 with different sizes two ends of the engaging block 5 are respectively formed with a first and a second engaging sections 51 52 the engaging block 5 is further formed with an oblique travel slot 53 a bottom of the engaging block 5 is disposed with a pivot section 54 pivotally connecting with the fixed jaw 2 accordingly the engaging block 5 can be swung left and right about the pivot section 54 a front section of the grip 7 is disposed with a first locating point 71 a second locating point 72 and a travel slot 73 the grip 7 is pivotally connected with the fixed jaw 2 each end of the switch button 6 is formed with a slide rail 61 bridged between the travel slot 53 of the engaging block 5 and the travel slot 73 of the.