If you are talking about the flexible pipe that goes from the heater tube to the walkthrough vent then it is 60 mm o/d diameter. I replaced mine with a silicone pipe from ebay.
Block it up and chuck the pipe, you wouldn't want to waste heat on back seat passengers. I made a stainless cap for mine.
Well, I have blocked of the end from the heater duct too. I have connected the walk through vent to my Propex which is sited under the r/r bed with the silicone tube mentioned for the bent bits and a length of 60mm exhaust pipe for the straight bit.
Think the flap actually works. Suffering from the heat riser/exchanger interface problem and may have to resort to self tapping screws to stop it keep popping off!!
