Böhm Stirling Technik Stirling Engine HB28

A beautifully engineered Stirling engine that runs really well. The only reason I didn't give it 5 stars is that, when rotated by hand, there is a grating/grinding noise that's hard to locate. A single-cylinder Bohm engine is very smooth when rotated by hand.I have I contacted Bohm about this, and they have said that this is normal for the engine, and that it requires running in. I'm not convinced of this, but I have only run it a few times so far, so we'll see.The other minor issue is the burners. There are 4 of these, and keeping them in step as far as the flame size and burning time are concerned is tricky. I think a single burner covering all 4 cylinders, or at least a single meths reservoir, would be an improvement.