to the invention adopts a sequence of non helical grooves 121 where the first groove 121 p and the last groove 121 d are identical but different to those 121 c arranged between the two these grooves are parallel and have a plane of symmetry or a median plane perpendicular to the longitudinal axis of the pin as illustrated these start and end grooves 121 p and 121 c of the first grooved portion 120 intended to receive the ring adopt a different pitch and a non symmetrical profile this profile has a rib depth and a height equal to those of the profile of the grooves 121 c but with a different slope on either side of the bottom of the groove in this way moving away from the head the slope joining the upper edge of the groove 121 p is more progressive than that starting from the bottom of the groove to join the second upper edge according to one preferred embodiment the width of the grooves 121 p and 121 c is equal to 1 5 times that of the grooves 121 c the same applies for the groove 121 d whereas said slopes on either side of the bottoms of the grooves 121 c are equal in order to enhance the behavior of the countersunk head 110 during fitting to prevent the so called umbrella deformation phenomenon the head has prior to fitting a slightly convex top surface according to one preferred but non limiting embodiment the grooves are preformed by rolling according to a further preferred embodiment the ring is made of 6000 series aluminum alloy and the pin is made of.