Did you install the fitting dry? Will you need to remove it in the future? I would first try teflon thread sealant, we use it on coolant, hydraulic, and oil fittings all the time and it seems to hold up to heat pretty well, and with a water-cooled header/manifold it should hold up. Next I would try loctite if thread sealant doesn't work. As a last resort if you won't have to remove the fitting.... jb weld. hope that helps!