Sorry everyone, you are absolute right. It was not so good idea to put one line in the forum and expect that everyone who want to help me knows about that. It is a little rude from me. So my apology for that. Let give you a little introduction of myself. I’ am a Dutch diver and make some little repairs (change battery’s, O-ring’s etc etc) on dive watches and dive computers from my divebudy’s. FYI Citizen dive watches are very popular by Dutch divers. I’am the owner of a citizen JP-1010 (caliber c500) dive watch for more than 14 years. I wear it every day and it did assist me as a backup (I also have a suunto dive computer) by a few hundred dives. So it is very frequently used and you can see it on the outside. So a bought a second hand one, changed the battery, reset it and it works fine. But I also want to replace the o-ring on the case back. For most dive computers I could find a service manual with the exact specification (hardness, type, shore, size etc etc) but I could find any specification about the caseback 0-ring of the citizen jp-1010 dive watch. I find the C500 repair manual on this side, thanks for that, but also there is nothing about the O-ring. So maybe someone on this forum can help me. Thanks in advance.