Generation 17 improved sealing around the pistons. Previous models used O-rings because they are easy to use but they removed all lubrication and begin to bind. Custom Delrin rings were made. The secondary expansion chamber piston was difficult because it has internal flow channels for the combustion gases and it supports the valves.
The video below shows the improved rings. The animation was done in SolidWorks using the animation wizard. Also shown in the video is the reciprocating motion of the double piston. Another improvement in this model is the coupling of the two engine halves. The fixed partition was redesigned and two ring gears were added. Previously, the engine halves were joined by a common main bearing. That guaranteed the alignment of the crankshafts but not necessarily the coplanarity of the fixed partitions. This improvement made the assembly easier.