Abstract: A gas spring for movable members such as vehicle deck lids in which a piston assembly carried on the end of a rod slides back and forth in a cylinder to expand and contract the spring. The cylinder is sealed at the head end and also around the rod and is filled with a combination of oil and gas under pressure. The piston assembly is such that the passage of oil and gas from one side to the other is restricted as the spring is expanded by extending the rod and this produces a dampening action but the restriction is eliminated when the spring is contracted so that there is virtually no dampening effect. The head end of the cylinder is closed by a plug which includes an inner portion carrying an O-ring to form a seal between the plug and the cylinder and an outer portion with peripheral teeth which bite into the cylinder wall and hold the plug in place.