I have been busy for the past week or so making a Stirling cycle engine. It is the Coffee Cup engine designed by Jan Ridders. It is powered by the heat from a coffee cup of hot fluid. Or an ice cube sitting on the top plate!
The piston is made from graphite. An interesting material to turn. Surprisingly tough, accepting a 3mm internal thread. And presumably self lubricating. Machining it produces black, pervasive dust. SWMBO is not impressed, since my CNC lathe sits in our living room. I might get marching orders for the lathe as a result of this one.