Carl the exhaust pipe port in the head is tapered to make a gas tight interference fit.
If the port is worn through a previous rattling pipe, the pipe can be swaged up at an exhaust centre with a cone. I fit mine with a finger full of Halford's exhaust assembly paste smeared around the fist 1/2"as belt and braces. Ron
Like Dave I used the silicone sealant. I had some left over from fitting the flue to my woodburner which is tolerant to high temperatures.It seems to do the job.Not sure how easy it will be to dismantle though when the time comes.