Thank you. I’ve put in a request to order an extra tail cap to be sure. When turning the cap on the threads it feels as if the tolerance of the threads has been set larger. It’s as if the cap can float on the threads with a fair amount of play/wiggle. On my SC5w MK II it had just a little bit of that but when you got to the o ring it would snug up to it with some resistance. With the SC600Fd Plus IV it feels like the o ring is too small or the cap was cut too much on the inside. It doesn’t feel like a tight seal at all. There is no resistance. When the cap is fully screwed on, despite the feel of poor o ring contact, it will stay secure. It just doesn’t take much at all to unscrew it from the tightened position. The threads are greased well.