What's the secret material to stop an exhaust leak btween the pipe and the head flange. My FMF pipe fits to sloppy for the O-rings to seal it.

 

Get some of the red high temp RTV sealent and put that around the flang and also where the pipe and silencer come together and this will stop your leaks.

 

I use expansion chamber sealer. You can get it at any motorcycle shop. It's kinda putty-like, you peal some off, wrap it around what ever you want to seal, put it together, wipe off the excess. Make sure everything is cleaned thouroughly before you start.
It does sound like you need a new pipe though.