I was on the lookout for an old hot-air engine for some time and eventually found this nice Bing twin cylinder model.
It is a twin cylinder beta type. Each side has the displacer and power piston inside the same cylinder. The power piston has a hole in it for a rod driving the displacement piston behind it. The power and displacement pistons are at 90 degrees phasing to each other. The two cylinders are at 180 degrees, because the pistons are only single acting.
The two hot ends are inside a firebox containing the burner. The cold ends are open to the atmosphere and have cooling vanes. The power pistons are approx 14mm bore x 13mm stroke. The displacement pistons are approx 11mm bore x 22mm stroke.
As acquired it was in non working condition, with one power piston disconnected and stuck inside it's cylinder. This has been freed up and reassembled. The original burner is missing so I have temporarily improvised with two small glass meths burners attached to a steel tray with silicone glue. The paintwork was in poor condition and may not be original anyway. I repainted the firebox and chimney with VHT.