A helical LSD comprising: a casing with a cap rotatable about an axis; a differential gear set including sun gears (23, 25) and planetary gears (17, 21) in mesh, both housed in the casing and being helical gears to create thrust force that presses the planetary gears (17, 21) onto the casing to limit differential motion between the sun gears (23, 25); the cap comprising: a clutch member combined with any of the sun gears (23, 25) and including clutch teeth; a clutch ring including a ring portion including clutch teeth so as to mesh with the clutch member and one or more tab portions being unitary with and projecting radially outward from the ring portion; a cap main body fixed on an axial end of the casing, the cap main body having a first face in contact with the casing and a second face axially opposed to the first face; a receding portion of the cap main body receding from the first face toward the second face and being so dimensioned as to receive and allow the clutch ring to be axially movable, the receding portion including one or more dock portions respectively receiving the one or more tab portions; an opening opened on the second face characterized by that the opening being in spatial communication with any of the dock portions to allow access to the one or more tab portions and the cap further comprising: side faces of the one or more tab portions, each of the side faces inclining toward the second face; inner faces of the dock portions inclining toward the first face so as to butt against the side faces in a circumferential direction to axially press the clutch ring toward the clutch member; and one or more base portions each having a shape capable of having contact with the casing in continuity between adjacent dock portions. a casing with a cap rotatable about an axis; a differential gear set including sun gears (23, 25) and planetary gears (17, 21) in mesh, both housed in the casing and being helical gears to create thrust force that presses the planetary gears (17, 21) onto the casing to limit differential motion between the sun gears (23, 25); a clutch member combined with any of the sun gears (23, 25) and including clutch teeth; a clutch ring including a ring portion including clutch teeth so as to mesh with the clutch member and one or more tab portions being unitary with and projecting radially outward from the ring portion; a cap main body fixed on an axial end of the casing, the cap main body having a first face in contact with the casing and a second face axially opposed to the first face; a receding portion of the cap main body receding from the first face toward the second face and being so dimensioned as to receive and allow the clutch ring to be axially movable, the receding portion including one or more dock portions respectively receiving the one or more tab portions; an opening opened on the second face characterized by that the opening being in spatial communication with any of the dock portions to allow access to the one or more tab portions and the cap further comprising: side faces of the one or more tab portions, each of the side faces inclining toward the second face; inner faces of the dock portions inclining toward the first face so as to butt against the side faces in a circumferential direction to axially press the clutch ring toward the clutch member; and one or more base portions each having a shape capable of having contact with the casing in continuity between adjacent dock portions. side faces of the one or more tab portions, each of the side faces inclining toward the second face; inner faces of the dock portions inclining toward the first face so as to butt against the side faces in a circumferential direction to axially press the clutch ring toward the clutch member; and one or more base portions each having a shape capable of having contact with the casing in continuity between adjacent dock portions.