I have set the gap between spinner and cowl to approximately .175" 
all around. I'd like to close in this gap a bit with an additional 
1BID on the face of the cowl. So, in everyone's opinion, how small a 
gap would be too small and result in eventual contact due to engine sag?

Angier,

 

Consider that the engine also moves around from torque applications.

 

Think of the problem as diagrammed by an H, where the left vertical (L) is the distance between the elastometric supports of the engine, the right vertical (R) is the diameter of the spinner and the horizontal (C) is the crank length (distance from the mount to the spinner).

 

If the engine sags, the displacement, as measured by the leg of a right triangle, is proportional for each similar triangle formed from the legs of the H as defined above.

 

Suppose L=15, R=10 and C=30 (measure for yourself).  Let's say the engine sagged or was displaced 1/2 inch as measured at the spinner.  That is about 1/4 inch at the bottom mount in the same plane (gee, I wonder why my soft bottom mounts are shimmed 1/4 inch?).  How far would the bottom of the spinner move back?  The similar triangle would have a leg of 1/6 inch but the triangle thru the center would make it 1/12 inch.  Oh, also the center back of the engine moved a bit aft when it sagged - about 1/8 inch (maybe less because the back of the top mount moved forward, too).  So, an EWAG of 5/24 inch is .21 inches or so of spinner edge movement in the above example.  Hmmm, perhaps the gap is a bit tight - maybe take out a washer to move the spinner back plate a bit forward (.0625) or fly in primer and fix the cowl scratches, if any, later.

 

BTW, the exhaust pipes move around in a similar fashion.