For the lower helix plunger (fig. 9-15, view A, and fig. 9-13), the effective stroke (injection) always begins at the same time, regardless of where the plunger is rotated because the top end that closes the ports is flat. The end of injection can be varied because of the sloping design of the helix. Injection has a constant beginning and a variable ending. This type of plunger is used in pumps marked timed for port closing.

For the upper helix plunger (fig. 9-15, view B), the beginning of the effective stroke varies as the plunger is rotated because the top edge, which closes the ports, is sloping. Thus, the beginning of delivery is variable and the ending is constant. Plungers of this type are used in pumps marked timed for port opening.

A third type of plunger (fig. 9-15, view C) has a variable beginning (upper helix) and a variable ending (lower helix) design. Rotation of this type of plunger varies both the beginning and the ending of delivery of the fuel.