I have the James rotary cowl--front “face” has a 13.75” diameter.  I also have the Vans 13” spinner that is already cut to fit my CATTO prop.  This leaves about a 3/8 lip of cowl showing all the way around the spinner.  Take a look at the picture.

I know any amount of lip will cause drag, but is this enough to warrant a new spinner?   UHS makes a 14” one that would probably do the trick.

Chris