When referring to maximum dial thickness - we are actually referring to the TOTAL thickness of the clock face (to include any material the face may be mounted/attached to). It may help to give an example as well (which will be provided below the suggested steps)...
The maximum dial thickness indicates the length of the threaded center shaft portion of a quartz movement. While the center shaft itself is much longer than the threaded portion, the threaded portion is the portion which must protrude through the clock face in order to secure the hex nut (and thereby secure the movement). NOTE: This does not account for movements secured with a threaded bushing (where dial thickness requirements must be more exact).
Suggested Steps to Determine Maximum Dial Thickness:
Step One: Determine the thickness of the clock face itself.
Step Two: Determine the thickness of any mount board that the clock face may be attached to (if applicable). Some clocks may not feature any sort of mount board. If a clock case should not have a mount board, then only the thickness of the clock face would be required.
Step Three: Select a dial thickness that will accommodate the resulting total of the dial face + the mount board thickness. Once again, if no mount board is used, then simply use the thickness of the clock face as the total thickness.
Example - I have a metal clock face which is 1/16" thick. It is attached to a 1/4" piece of plywood veneer. The maximum dial thickness would be equal to 1/4" + 1/16". Therefore, the customer should select a shaft that has (at least) 5/16" maximum dial thickness (or greater).
If the maximum dial thickness is greater, customers can space the movement back within the case with rubber gaskets or a spacer block (a simple, square block with a center through hole for the center shaft).
For Example: Let's say that the maximum dial thickness is 3/8" - but our clock face/dial board combined thickness is only 5/16". We can use a spacer to help take up the excess shaft length. In this example, we would require a spacer that was 1/16" in thickness. Whether using rubber gaskets as spacers, or scrap wood blocks with a center thru hole, the result will still be the same.