A jar is disclosed comprising: an outer housing; an inner mandrel at least partially disposed telescopically within the outer housing to define a fluid chamber between the inner mandrel and the outer housing, the fluid chamber containing fluid and being sealed; a valve disposed within the fluid chamber, the valve having a restriction surface facing, during setting, a cooperating restriction surface on one of the inner mandrel and the outer housing and a pressure surface facing the other of the inner mandrel and the outer housing, the valve being movable between a seated position in which the valve seats against the other of the inner mandrel and the outer housing and an unseated position; the cooperating restriction surface cooperating with the restriction surface to set the jar for a jar in a first direction; the pressure surface being exposed to fluid within the fluid chamber when the jar is set and pressuring up for a jar in the first direction; the pressure surface being indented to cause uneven radial movement, in a plane perpendicular to a jar axis, of the restriction surface towards the cooperating restriction surface when the jar is set and pressuring up for a jar in the first direction; and jarring surfaces on the inner mandrel and the outer housing respectively for jarring contact with each other during a jar in the first direction.