The not quite right guy is back...... the OP said the rocker was making contact on the inside edges (a photo would help) of the cap. “If” by that he means the narrower part of the cap where the threads are, It may be that a thinner gasket gasket (which brings the narrower part of the cap down and away from the nut) will do the job, or he may have to get some material skimmed from inside the cap. If you compare the new caps available today, to OEM, you can see a difference in that area.
It definitely helps to have this stuff right in front of you when trying to figure it out.... which I don’t at the moment, so risk of error increases.
Not to worry... if I ‘m out to lunch on this one, Robert will have me wear the pointy hat at the next gathering.
I took these back when I was assembling the Mongrel Comet engine, so going from “memory”.
First photo (apologies for the quality) is OEM, second is new reproduction. The area between the red and black marker is cut on a taper. The original is more concave in that area. Not a big difference, but every little bit helps.