Found some more pictures, and, since Nathan amongst others (me included
) likes to see the process of making parts, I will not withhold them.
So this is how I aligned the piston rod with its ring head during soldering. Note the small ring of solder paste around the piston end. I found the result to be rather sensitive to the amount of solder paste. Too little, and the solder did not fill the joint properly. Too much, and the solder spreads all around the correct place. What you don´t see in the picture is the small stub (1.5 mm, 1/16" long) at the end of the rod inserted in a corresponding hole in the ring. For maximum strength the fit should be loose to leave a ring shaped gap for the solder to enter. I made the hole about 0.15 mm (6 "thou") oversized.
The next pic I just shot to show how the solder was added. I´m not so good at adding the right amount manually for these delicate jobs, so I bent a piece to fit around the rod and stuck it in the paste. When the water in the paste boils those small pieces of solder tends to move away from the corner, so have some pointed object at hand for pushing them back.
Stefan