Now the coupling has to be "fitted" to the shaft. These coupling ship a tad undersized for the SAE shaft tolerances. This allows a competent shafting shop to "fit" the shaft to the coupling. The proper fit for a straight coupling is a light press fit or light interference fit. This means it does not just "slide on" and requires some light tapping, or heat to expand the coupling while it is installed over the shaft.. Getting this level of fit can be time consuming.
The two shims are placed in the "splits" to make the bore ID the same at both ends before "fitting" begins.