One tip I have not seen mentioned, is from an old issue of "Twitter" (GSOC news letter) from the '70s. I personally tried this on my RGS primary which was warped, producing a 1/8" gap.
First remove any high spots and clean the threads. Next, bolt the inner and outer cover together at approx. 25 ft. lbs. Place in an oven preheated to 400 degrees. Heat for about an hour and let cool. The gap disappeared.