I just put this on the forum, I tried a lot of different plugs but the ones I give on this link are by far the best fit. Their the flushest and best diameter in terms of the smallest whilst maintaing water tightness. The only let down so to speak is the rough finish but this can be easily smooth out with glass paper and built up with OEM touch up paint spray.

http://www.xoutpost.com/x5-e53-forum/...ion-plugs.html


I had the X -Pipe made up by an exhaust manufacturing and design company. These guys are UK based but I have to say the best I have come across, they do my bike exhausts. But specialise in performance car exhaust build and design.