Grumpy does have a point and I don't have any experience whatsoever brazing aluminium. Only ever brazed copper pipe fittings together using flux and a butane torch. It would be quite difficult to apply heat evenly in this case because there is not much room to manoeuvre. I wonder if a fastener like a stud might do the trick instead? There is still a bit of mount left near the cylinder wall to tap some threads.
