A closure pin extends through aligned openings in first and second spaced apart eyes at the open end of a substantially U-shaped clevis. One end of the closure pin is threaded and mates with threads in the opening in the second eye. The closure pin includes a head portion and a radial flange at its opposite end, and it carries a releasable lock member. The releasable lock member includes a pair of detents which are positioned in radial slots in the flange. A spring biases the lock member into a position in which the detents extend axially beyond the radial flange towards the threaded end of the pin. A plurality of lock ramps are provided on the outer face of the first eye. The ramps are spaced circumferentially apart. Each ramp includes a first end which is substantially flush with the outer face, a second end spaced circumferentially from the first end and axially outwardly from said face, and inclined surface extending from the first end to the second end, and a second surface at the second end extending out from said outer face. As the closure pin is rotated to connect it to the clevis, the detents contact and move up the inclined surfaces of the ramps and by such contact are moved against the force of biasing spring into the slots, allowing the closure pin to be rotated. When the closure pin is connected to the clevis, an unwanted reverse rotation of the closure pin is blocked by contact between the detents and the end surfaces of the ramps. The closure pin can be easily disconnected from the clevis by a user retracting the lock member, to retract the detents into the slots of the annular flange, while at the same time rotating the closure pin in a reverse direction.