That's one thing I really like about modern engines. O-ring seals do a way better job of preventing leaks than old style gaskets I think. Though I did have one little coolant leak at the timing cover when I first put my engine together. I think it actually fixed itself on its own though with a heat cycle or two. Or maybe I pulled the cover and swapped the o-ring, I can't remember. Either way it was easy to fix and it's been nice and clean since. The valve covers do seem to have a problem leaking around the plug wells though. There's no real clamping force in that area, so I'm not sure the o-rings get good compression to seal there or not compared to the perimeter.