Why is it so important that the spinner and prop blade not touch? Does it somehow weaken the prop or set up some kind of detrimental vibration that can cause the spinner or prop to loosen and come off?

Props flex. If that flexing is constrained by touching the spinner or any other hard point then the stresses will be more concentrated at that point and the prop could potentially fail. For that reason it's considered best to leave a 1/ 32 or so clearance to ensure that does not happen.

Thanks -I always wondered what the reasoning was behind that idea and I'm just now having to trim an aluminum spinner for clearance of a ma 3 blade for my TT Rare Bear. Now I gotta figure out how to balance the spinner after trimming for the prop

If you have one of those Dubro prop balancers with the sliding cones that sit either on the large thin aluminium wheels or the older magnetically suspended one they work great for even small stuff.

But the real key is to layout the cutouts perfectly so they are exactly 120 degrees apart and are the same shape. Here's a trick for you. Draw up a single prop cutout in some business card stock. Try it out to make sure it fits the prop with the right clearance. If you're clever and careful you can even make it fit back under the front lip for a neater look. Now cut out a 1/4 inch wide strip of paper and wrap it around the base of the spinner. Put a little pencil mark exactly where the end of the paper touches the wrap. Spread out the paper and measure the length to within 1/64. Divide the total dimension by 3 and mark little pencil ticks on the paper as accuratley as you can so your wrap part is exactly divided into three segments. You don't need to sweat a few thou here and there in the pencil line but try to get it as close as you can aiming for the center of the pencil mark as well as you are able. Re-wrap the paper around the base of the spinner and with a knife cut a shor little 1/32 long mark into the material of the spinner at the end of the paper and the other two pencil ticks. That's your spinner divided into 3 equal parts now. Now offer up your cutting template to the spinner and mark the cutouts. It doesn't matter what edge of the cutout you use as a locator but obviously keep them all the same. Now carefully cut and file out the cutouts and you should have a reallly nice fitting spinner.

Howwzat?

Excellent tip - Thanks!