I've spent a lot of time (more than I'm logging here) over the past days filling and sanding the inside of the lower cowl in preparation for paint & today I was able to create the hole for the cabin heater shroud inlet pipe to pass through. Starting with a small hole approximately centred before the shroud was completed by using some cardboard tube as a guide, it was a case of sneaking up on it slowly by making small adjustments with a Dremel sanding drum then try the cowl in position, mark, remove, trim, re-install & repeat several times. In the end the hole is 50mm in diameter with the pipe being 38mm giving 6mm clearance all around, hopefully this will be enough to allow normal engine movement without the pipe hitting the cowl.
