ORIGINAL: karolh
With such a big round cowl you need some space between prop and cowl to get the max thrust effect from your prop and I would suggest using 3/4" as a minimum clearance.
Karol
<hr />Karol is right on. I would add that a big round cowl the largest dia prop with a lower pitch works good too. The cowl shape makes the space needed different. A pylon racer with a stream lined cowl kinda blends into the spinner...not much gap needed. Capt,n