The purists wont like this but you can use normal clear silicon mastic, as sold for many domestic purposes. Plenty around the pipe and inside the nut and round its thread and it 'squidges' to fill up any gaps. It also does not set hard so that the nuts can be unscrewed later if needed. I would never have guessed at this 'bodge' had I not seen it being used on racing sidecar outfits in the paddock after cylinder head replacements during races. I tried it at the joint where the front pipe joins into the rear pipe, which must be cooler than at the head, and it lasted for years, If you have the mastic lying around from some domestic job then it is an almost zero cost option. By the way, I specified 'clear' above as I have found that lasts for years in Mediterranean sunny conditions when used for bedding in clear plastic into aluminium hatch frames. The chaps I first saw using this were using some coloured stuff, obviously left over from a domestic job. My experience is that for the use in the Sun any pigment reduces the lifetime to about a year before it looses its elasticity; the clear lasts at least ten years.
