Hello Mark and net. I looked at your pictures http://mywebpage.netscape.com/n886mj/photo.html and it appears to me(opinion) that the crack originated at the cutout for the prop. It is important that netters understand that when cutouts are made in aluminum, that localized areas of stress developed in the sharp freshly cut aluminum. Care must be taken to relieve that stress by rounding and polishing the edges in the area where the cuts were made. If this is not done, cracking from the stress area will developed rapidly.
I have never purchased an expensive spinner, and have never ordered one where I could request different sizes to accommodate differences in prop thickness. Although I have lost one, it was because of backplate failure. I have had a second backplate crack as well. I agree with inspection requirements of both the spinner and backplate. In 20 years I have never had a fwd bulkhead. Another important issue is keeping the alignment between the backplate and the dome. if the screw attach holes are not drilled correctly, the spinner will wobble, which is an out of balance condition. The worse the wobble, the more vibration and the more likely and rapid a failure will be. My screws are NAS1032 and I use floating nutplates on the back of the backplate. Screws will gall the spinner. Most production aircraft use a plastic washer under the head of the screws. Orma Southfield, MI N110LR Tweety, old enough to drink this year Flying and more flying, to the gathering or bust http://www.kr-2.aviation-mechanics.com/
