Yea and watch the raybias. Suggestion: while testing the material AO, turn off all other lighting effects (Diffuse_Value = 0, Specular_Value=0, Reflection_Value=0) and plug the AO node into Alternate_Diffuse.

It should look something like this. Using this technique, you can easily see what the AO is doing, and not confuse the issue with other shader stuff.

Having done this test with the AO node, I'm now certain this has to do with ray-tracing a displaced surface.

Maybe you should completely separate those rings and actually model them. Just use the displacement for rivets. Right now there are tons of AO artifacts. I didn't bother adjusting or tweaking or using higher render settings, so its pretty bad. It may not be possible to eliminate them.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)