Thanks Per
I struggled to get the engine running for more than 30 seconds, so took a long look at places where there might be excess friction or weight. I replaced the steel con rod with aluminium, thinned down the displacer link rod, shaved a bit off each side of the flywheel rim and a bit more off the hub, and made the securing holes for one of the bearing standards a bit bigger so I could adjust alignment a little. While the engine was stripped down I tested the displacer in hot water again and found a tiny leak at the silver soldered cold end. I'd used a little JB Weld when finally fixing the displacer rod and that was holding up fine with the heat so I used some of that to fix the small leak.
After much fiddling around with flame position and using a few pieces of chopped up beer can as cooling fins I've finally got it running as expected - reasonable speed and for as long as I was prepared to hold the ali in place with a screwdriver! In this video it starts to slow down but when I press the screwdriver a little more firmly against the displacer cylinder and the back of the frame it speeds back up.
So, I need to make a little radiator. I can't slide it onto the displacer cylinder since that has a lip at the hot end so I'm probably going to have to clamp something to it (I don't want to upset things by soldering). I guess I could make a small water tank in two parts or some simple upright fins, the latter would probably be easier and lighter so less likely to affect the cylinder if I can find something to do the job.
