Referring back to this post:
I made the two piece OPRV:
[Linked Image from]
The bearing ball sits against a rubber backed washer, held in by a spiral ring. A hollow set screw sets the pressure on the ball and allows the oil through to the relief hole.
I had to use RTV on the washer because the diameter of the rubber was the same as the bypass slots around the ball. Making the slots slightly less deep will let the rubber seal against the body. In any case there is no leakage with the ball seated.
Being two piece, the body threads can be sealed in the case without potentially blocking the relief hole. A hex is provided on the body for turning it in.
The cap is just a cover.