I had some bits and pieces left over from other projects and decided to make a "Frankenstein" engine, no real plans, I would make the pieces fit. I had 2 brass cylinders with 7/16 bores and a crankshaft that sort of matched. I found an old base I could adapt and lots of cut off's so off I went. It was to be my version of a twin cylinder spool valve engine with everything in brass apart from the base and the 7BA screws for the cylinder. I got to the point yesterday where I had the body of the engine assembled but still to make the spool valves. The bores were reamed out, pistons polished etc and the whole thing turned over just fine. It was running (by hand only) dry so I added a couple of drops of light oil to the bores, my nice free engine got tighter and tighter ! I took it apart and the bores were now scored along with the pistons and the slides under the pistons. I thought something must have been binding so I cleaned everything up again to the point where the pistons were as loose as I dare to go, assembled it again and it turned perfectly smooth. I turned it for a while just by hand and all looked good so again I added a little oil, it instantly tightened up and the bores/pistons were scored again ! I dont know why this is happening, is it a "brass on brass problem" ? is the oil causing it to bind and pick up ? I'm only using light machine oil.