I know this won't work, but why not?

If you could drill three equal holes sideways through the rotor between the e shaft hole and the apex seal. You would cut the weight quite a bit and if it balanced it wouldn't vibrate. You could put plugs in just the outsides of the hole and have the center hollow.
Or you could just leave hollow through the rotor.
Just something I thought of. I am sure someone has tried this somewhere.

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I don't know how to get my diagram to stay as I made it so use your imagination

 

It has been done but not for weight savings.

http://www.hurleyrotary.com/seals/tips/lubesys.htm

 

That is cool, but I meant through the side of the rotor.
Parallel with the e-shaft hole.

 

The rotors are not solid. They are sort of hollow and fill with oil. Many rotary shops sell lightened rotors, they lighten them by removing material from the sides of the rotors.

 

I thought they were solid. Thanks for the info. I am rebuilding my 86 engine this winter so I will have a more intamate knowledge of the rotors then. Thanks.