Abstract: Method for longitudinal cutting of a wall of a fibrous tubular product in a step shaped profile. The apparatus comprises two parallel main blades which form radial cuts into the wall, slightly displaced from each other. One cut is made radially inward from the outer surface; and the other cut is made radially outward from the inner surface. Each cut extends toward the center of the shell wall to a depth approximately one half the thickness of the shell wall. A third cut is made within the shell wall perpendicular to and joining the other two cuts. These three cuts form a smooth step shaped cut through the wall and facilitate installation of the shell around conduit while minimizing heat loss and mechanical stresses within the wall.