How did you tighten the joints? by hand? Can you put a wrench on it and get another 1/4 or 1/2 turn? Threaded joints are probably standard pipe thread which has a taper so the more you tighten it the better it seals.

I wouldn't worry about the leaks. They only leak when in use and the joints are not under a lot of pressure so they will not leak as much as they do when pressure tested.

Yes Paul, hand tight! I guess I could tighten them a bit more, but the problem is that it would have to be a full 360 degrees turn - these flexi-risers are not at all that flexible :) - and they must remain in the positions I have placed them, where the sprinklers should be. Did I mention I have only used ells :P ?

Taking one more confirmation for my thoughts and going to bury them. Come, come!

Ah, actually i was able to turn them once, even twice. Now leak free :)