6 were formed from an aluminum rod such as one defined byjis a 3003 it appears from table 1 that when a pipe 6 having an outside diameter d 1 sup u03c6 of 8 mm and a thickness t of 1 mm is chosenand when a clearance c if 1 5 mm is used and the pipe 6 is subsequentlyaxially compressed to form an annular ridge 7 the resulting annular ridge 7 and the mating annular recess 5 provide a coupling strength of larger than 0 4 kgm to thereby retain the union 1 against rotation around the pipe 6 thus the nut 9 can be threaded by hand on the union 1 without manually holding the union 1 until the tightening force applied on the union 1 exceeds the foregoing coupling strength whereupon the union 1 corotates with the nut 9 then the nut 9 and the union 1 are firmly tightened together by suitable tools such as wrenches the final tightening force required for the pipe 6 of the foregoing dimensions is 1 5 sup u00b1 0 5 kgm as shown in table 1 it appears from table 1 the coupling strength increases with an increase in the clearance c the nut 9 is made of metal and machined into the shape shown in fig1 thenut 9 includes a non illustrated wrench head and an axial central stepped bore composed of an internally threaded portion 10a and a small diameter portion 10b separated by an annular shoulder 11 the shoulder 11 is engageable with.