Stirlingengineforum.com

From what I see from Internet, there is not heat barrier between hot and cold ends.
In the transition zone, it will lose thermal energy.

I am thinking,
use a long cylinder,
long displacer,
add a bypass pipe between hot end and cold end as gas pass.
the displacer will not leakage gas.
in between hot and cold ends add a section of heat barrier cylinder.

Nothing new there, but it's a good idea to isolate the hot and cold ends of the displacer cylinder. In practice the use of stainless steel for the hot cap, joined to an aluminium cold end, with a thermal dam at the cool end of the hot cap. A thermal dam is a short section of the hot cap with reduced thickness of metal, the reduced amount of metal increases the thermal resistance, for example the hot end may be for example 1.5 mm thick, where as the dam area might be 0.75 mm thick, also a thick insulating gasket between the hot and cold end helps a bit more.
You will have noticed that for a good performing displacer it should be about three times it's diameter long. A certain amount of regeneration takes place along the surface of a displacer of this shape.
Ian S C