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I have another crackpot idea. Suppose we use conical springs to increase heater and cooler efficiency and maybe help the displacer move along, too.?
The springs would be attached to the displacer, and they would contact the cylinder housing at all times due to very low tension. When the piston reaches top dead center, the spring would be folded flat and not interfere much with the stroke length.
I think the increased internal surface area would be helpful no matter what the design may be, and could be added easily at that. The material used is up for debate. Copper would be nice, but only if using helium.

Don't know if it would work, but the spring at the hot end would last until the motor got up to heat for the first time, it would then be stuffed. A spring made of copper would last until it work hardened, then break. The spring at the hot end could be replaced with one at the outside of the cold end, steel or bronze OK.
On my Ringbom engines I fit a spring on the outside of the displacer, mainly to reduce the noise, but I suppose it gives the displacer a bit of help. To reduce the impact noise on the upward movement I place an O ring on top of the displacer, inside the cylinder. My first free piston motor failed when the displacer tried to escape through the hot end of the cylinder, As soon as a crack formed the motor stopped of course.
Worth looking at, I think a Ringbom motor (Tapper) would be the type that might benifit. On other motors its just a bit of extra load. Ian S C

Perhaps I could use a copper coated stainless spring for the hot side, assuming it is possible to coat stainless with copper. Nah, probably would flake off in the high temps. I was mostly trying to get more heat to transfer to the working fluid via the springs. I normally would try to use metallic foam for a displacer piston, but the makers of such hi-tech products refuse to give me the time of day. So I am looking for ways around this. It just would be nice to have a displacer piston as a regenerator, also.
I forgot that copper can work harden....I should have known better. At least I can employ this on the cold side if I have to. I have capillary copper tubing I could cut into beads and string them upon the springs like a necklace. The hot spring can be 316 stainless with copper beads installed around the wire. This may not work at red hot temps, but should allow a lower operating temperature.
Here are copper necklace beads manufactured commercially.
http://www.amazon.com/gp/product/B002UB ... B004XW1X8Y