Abstract: A self-retaining collar for a pin of a two-piece fastener has a projection or detent from its inner surface for engaging locking grooves of the pin. The pin has a head and a shank, the locking grooves being formed on the shank. The fastener holds two or more workpieces together in tension by inserting the pin through the workpieces until the head contacts one side of the workpieces and the locking grooves extend through the other side. The shank has pulling grooves located on the side opposite the head. Once the pin is through the workpieces, the collar is slid over the shank until the detent engages the lands of one of the locking grooves. The collar will thus stay in place around the locking grooves even when the force of gravity tends to pull the collar off of the grooves. The collar is then swaged into the locking grooves while the workpieces are placed in tension by pulling on the pulling grooves of the pin.