PURPOSE: To improve the hardness of a nut seat periphery and increase its durability witout thickening a mounting area by finishing the nut seat of the mounting hole in a disk used for mounting a car wheel through coining fabrication.

CONSTITUTION: In the disk 5 of a car wheel 1, the nut seat 7 of a mounting hole 6 to a hub bolt is pre-cut into the configuration of the tightening surface area of nut which is used to mount the disk on the wheel hub, i.e. a counter-sunk seat shape which has the smaller curvature R1 or angle α1 than thos of nut spherical and tapered surface areas. Then the nut seat 7 is coined using a press machine which has the upper and lower molds with the same number of pins as the mounting holes 6 on the disk 5 and with the same pitch arragement. This permits the nut seat 7 to be fabricated into a form which has almost the same curvature R2 and angle α2 as those of the nut spherical and tapered surface areas and improves the hardness of its periphery.