Stirling Engines, which can be Low-Temperature-Difference(LTD) engines, are definitely quite interesting in the sense that the desk toy versions can run on little temperature difference between the plates, therefore something like a coffee can easily get it running after a few spins. Looking more into them I decided to model a twin-cylinder one.
In Roblox Studio
I did try out the SurfaceAppearance for PBR textures though the results in Substance Painter and studio were vastly different with metal. Ended up settling back with Roblox’s normal metal and smooth-plastic with reflectance in the end.