There seems to be some confusion regarding what center rod is used with the various diaphragms. Hopefully this article, and the chart below will add some clarity to this issue.
On pumps 1/2” in size and smaller (NDP-5/DP-10/DP-15/NDP-15), regardless of the diaphragm material, the shaft length and center
disk diameter will be the same.
On the 3/4” and 1” pumps (NDP-20/NDP-25) the shaft length will remain the same whether you are using a rubber diaphragm or a PTFE diaphragm; although the metal pumps will use a longer center rod than the plastic pumps.
The center disks will be a smaller diameter when using PTFE diaphragms, as opposed to a larger diameter for rubber diaphragms.
The stroke is short when using the PTFE diaphragms, this destroke
is accomplished by changing the thickness of the air side center disk instead of shortening the center rod.
On the larger pumps the (NDP-40/50/80) PTFE center disk will
be a smaller diameter than the rubber center disks. In addition,
the center rod used with the PTFE diaphragm will be shorter than the center rod used with the rubber diaphragms.
You can use the following chart to help identify which center rods and center disks you have.