I cannot recall the specific oring now, but the information is readily available on Reef Photo's web site. The Nauticam port orings are much thicker. I would not even consider using one. E_Viking may have solved this problem. if you turn it fully counter clockwise, it locks again. Backing off just a little frees it up. I was not quite sure what he meant, but stopped in Ocean Optical, and Jack Connick showed me in Person. Many Thanks.