A couple of tricks I have read about over the years for sealing up headers: Soak the header gaskets in water for a while prior to installation. If you have the one piece header flange, separating each tube will allow them to move freely as they heat up and cool down. I would also consider going to the kinda header bolts that have a "c" clip which keeps them from backing out after they have been tightened. An inexpensive way of doing the same thing would be to use two flat washers between the header bolt head and flange. Bob Rapp NN, VA 70 Machine