No problem and I'll add that the port size of an ICE engine can be smaller than the valve diameter, and the port of a Stirling even smaller still. I'd bet you could be at 2-3mm, possibly smaller.Vincent, thank you for info. That's a useful little calculator :-) I just put in guess of 400 rpm until I can measure it and it came out at 4.9mm so pretty close to what it is (5.5mm). I guess with a 10" fan this is quite a slow running engine.
The smaller the port the more velocity the gas has. That, along with the far reduced surface area will help the gas maintain temperature.
I have a pet theory that there are other benefits to this far reduced port size in Stirling engines. The restriction in flow caused by a small port allows a slight heat pump effect to occur at the power piston area, before the primary pressure pulses from the displacer chamber drive the power piston. There is added compression towards TDC that helps maintain temperature, and an extra reduction in pressure that helps cool the air in the power piston towards BDC.
In conjunction with a power piston of low thermal conductivity, I think this helps maintain the right temperatures at the right time.
I'll stop ranting now, but if you try it please give an update.