Mike,

The 46 VX-M has a (blue) water-cooled cylinder head. The 46 VX-DF has a (purple) air-cooled cylinder head. In addition, the -M does not have the thrust washer on the crankshaft (forward of the front bearing) that butts up against the Turbax 46 rotor+spacer assembly.
The thrust washer is the same part that was used on the older 46 VRDF, so if you have one of those lying around, you can vulch the part off of it. Then all that's left is finding a cylinder head. I was able to get one from Irvine in the UK (since GP was not listing it as a spare part at the time).
Another option is ordering the 46 VX-DF from a source overseas. I have done that successfully to (from a shop in Singapore). The price (for a 4-engine order) was cheaper that what Tower was selling the engine for in the US.

Hope that "helps".


F900: I used to fly a scratch built Philip Noel Mirage 2000C (the one featured in the 2-issues article in RCJI a few years ago). After 175 odd flights, I retired the jet and it is now a hangar queen/gate guard in my shop. Busy working on a JHH Mirage 5F and a coouple Mirage F1 projects...