A roller for a mechanical fuel pump assembly with a rotating cam, the roller comprising: an elongate body extending from a first axial end to a second axial end of the elongate body over an overall length of the elongate body, the elongate body defining a first length that is less than the overall length; the elongate body having an exterior cylindrical surface defining a uniform circular cross section over a second length of the elongate body, the second length extending between a first plane and a second plane (A') and the first length extending between a third plane and a fourth plane (B'); a surface between the first plane and the third plane defining a first area of reduced cross section extending axially outward from the first plane to the third plane located axially inward of the first axial end; a surface between the second plane (A') and the fourth plane (B') defining a second area of reduced cross section extending axially outward from the second plane (A') to the fourth plane (B') located axially inward of the second axial end; and the cam defining a cam surface; wherein at least one of the first area of reduced cross section and the second area of reduced cross section comprises a profile having at least one radius of curvature; and wherein the roller is arranged to roll on the cam surface as the cam rotates around an axis, characterized in that the cam surface and the roller defining a patch width of contact area between the exterior cylindrical surface or the surface between the first plane and the third plane and the surface between the second plane (A') and the fourth plane (B'), and the cam surface so that the second length being 75 to 90 percent of the first length is configured for hydro-dynamic wedging and lift of the elongate body away from a shoe of the fuel pump assembly and while the at least one radius of curvature R is configured to relieve contact stress on at least one of a surface between the first plane and the third plane and the surface between the second plane (A') and the fourth plane (B'). an elongate body extending from a first axial end to a second axial end of the elongate body over an overall length of the elongate body, the elongate body defining a first length that is less than the overall length; the elongate body having an exterior cylindrical surface defining a uniform circular cross section over a second length of the elongate body, the second length extending between a first plane and a second plane (A') and the first length extending between a third plane and a fourth plane (B'); a surface between the first plane and the third plane defining a first area of reduced cross section extending axially outward from the first plane to the third plane located axially inward of the first axial end; a surface between the second plane (A') and the fourth plane (B') defining a second area of reduced cross section extending axially outward from the second plane (A') to the fourth plane (B') located axially inward of the second axial end; and the cam defining a cam surface; wherein at least one of the first area of reduced cross section and the second area of reduced cross section comprises a profile having at least one radius of curvature; and wherein the roller is arranged to roll on the cam surface as the cam rotates around an axis, characterized in that the cam surface and the roller defining a patch width of contact area between the exterior cylindrical surface or the surface between the first plane and the third plane and the surface between the second plane (A') and the fourth plane (B'), and the cam surface so that the second length being 75 to 90 percent of the first length is configured for hydro-dynamic wedging and lift of the elongate body away from a shoe of the fuel pump assembly and while the at least one radius of curvature R is configured to relieve contact stress on at least one of a surface between the first plane and the third plane and the surface between the second plane (A') and the fourth plane (B').