Allthough I am sure that a long tube between displacer chamber and power piston somehow affects engine performace, I agree that the most resonable explanation for the two minute prewarm time is as you say- related to engine mass and probably friction as well. In my case, the displacer itself is...
Thanks Longboy I really took a risk in making such a long path for the pressure variations. I estimate the combined length of the drilled tube between power piston and the displacer chamber to be at least 150mm long, and this length almost kept me awake some nights! I was quite worried that...
The final engine. The second picture shows it together with my first LTD. The engine runs at around 900 RPM for 20 minutes on a tank. Thanks for Reading this long story:)
Drilling tunnels. I brazed a longer shaft to the drill bit using the lathe chuck and tailstock as holders. Sorry for the blurry picture. The drillbit ended up about 120 mm long.