RE: Teeny Valentine Diesel
I have several of those superb little engines, and I've found that one of the old Cox grey reinforced nylon 4 1/2 x 2 props made for their .020 glow engines works pretty well on the Valentine .0061 when cut down to 3.75 in. diameter, and does move some air. This is a diesel, so the variable ignition timing allows it to turn a useful prop. I suppose that one of the old Cox .010 props of 3.5 in. diameter might also work well, but I've never tried one.
Thi is actually one of Ronald's larger "miniatures" - his smallest is the Nano-Bee at only 0.00037 cu. in. (yes, that is 0.006 cc!). That one runs on Ronald's custom-made 1 in dia airscrew with a 1/2 in. pitch!!
One point of which to remain aware - these really tiny engines tend not to last all that long in service, and this is not down to any manufacturing shortcomings. It's just that the very small combustion chamber volumes mean that piston leak-down has a far greater effect on operating efficiency and it only takes a very small amount of wear to make starting very difficult.
Adrian