For a direct-reversing slow-speed engine the fuel pump cam is a "double-lobe" (twin-lobe / reversible) cam: it has two identical lobes/inert faces that are geometrically disposed so that the follower can contact the same working profile whether the direction of rotation is ahead or astern. The cam is designed as two lobes offset by the reverse of the normal profile; one lobe serves the ahead rotation (the follower rides up the "ahead" flank) and the other serves the astern rotation (the follower rides up the "astern" flank). The flank rise is of the correct shape to produce a smooth plunger lift giving the required injection timing and quantity, and the dwell/top of the cam is the delivery-cut-off point. By axially shifting the cam (or by using two cams on a lever), the correct profile is brought into contact with the follower for the direction of rotation, so that the fuel pump timing is correct for either direction. The cam profile is thus a symmetrical/lenticular (fish-shaped) "reversible" cam with the rise and fall designed symmetrically so that, when the roller is transferred to the correct lobe, the injection timing (governed by the beginning of the rise) is correct in both directions.

Fuel pump cams are made of a hard, wear-resistant material (e.g. hardened steel or cast iron). The heat treatment during manufacture includes:

1. Normalising or spheroidal annealing of the steel to give a uniform, machinable structure before machining.
2. Case hardening / carburising: the cam surface is carburised (case hardened) to give a hard wear-resistant case (e.g. ~1.5-2.0 mm case) while the core is kept tough, followed by hardening and tempering.
3. Alternatively, induction (flame) hardening of the working surface of a medium-carbon steel, followed by tempering to remove internal stress.
4. Tempering after hardening to reduce brittleness and internal stress while retaining hardness.

The heat treatment gives a hard surface to resist the very high contact pressures and wear of the roller/follower, and a tough core to resist impact/fatigue and withstand the torque of driving the fuel pump.

In a reversible engine the camshaft is carried in a box/support and is axially moved relative to the crankshaft by the reversing mechanism. On engines that reverse by axial movement of the camshaft: the reversing gear (usually a hydraulic piston/hydraulic cylinder actuated from the control) shifts the camshaft axially through the reversing fork/lever so that the "astern" set of cam profiles moves into line with the cam followers while the crankshaft-driven drive gears remain meshed. Alternatively, for engines where each cylinder's cams cannot be shifted, the fuel pump cam timing is changed by rotating the cam relative to the crankshaft via a timing gear/index mechanism (the camshaft drive includes a mechanism to advance/retard the camshaft relative to the crank by the appropriate angle for reversal). The result is that the effective angular position of the cam (and hence the fuel injection timing) relative to the crankshaft is moved for astern running. On electronically controlled (camshaftless) engines there is no cam; the timing is switched in software.