A jar comprises an outer housing; an inner mandrel within the outer housing to define a fluid chamber; a valve disposed within the fluid chamber with a restriction surface facing, during setting, a cooperating restriction surface on the outer housing and a pressure surface facing the inner mandrel, the valve movable between a seated position 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.