How I dunnit...
I found that a 5/32” brass tube is a tight press fit in the Medallion NVA holes. The standard fine thread NVA from a postage stamp backplate is a loose fit in the tube. The first thing I did was take my calipers and measure across the sides of the venturi where the stock NVA goes. It’s 0.375” across. I cut a piece of tubing that width and then measured the venturi. The width of the throat is about 0.175”, so I marked 0.1” from each side of the tube. I then used my Dremel to cut out the center of the tube, leaving a thin strip between the ends. This made a nice carrier for the NVA barrel. Hmmm… the cuts were not too even. Next attempt I will take more time cutting the tube to ensure everything is square.
I then soldered the NVA barrel into the carrier, ensuring the brass strip was opposite the hole in the barrel. Afterwards, I took my jeweler’s files and smoothed and shaped the brass backing strip so that it was narrower than the NVA diameter to allow full air flow through the venturi.
Before installing, I took some coarse sandpaper and ran it lengthwise along the carrier to knurl the tubing so it wouldn’t rotate in the plastic. Then I just pressed it in with the hole pointing down. The hole’s a little off-center, but since it’s bladder pressure that doesn’t matter.
So, the first few times I tried to run the engine, it wasn’t starting. Seems the two Cox glow heads I tried were pretty weak. I put on a Merlin adapter and insert, and Bazinga! (©Chuck Lorre Productions, all rights reserved.) It seems a hair over-compressed, maybe. I may try another glow head gasket. But 18,650 or thereabouts seems pretty good. It’s a Medallion, not a TeeDee. Who knows what another gasket might gain or lose? I’ll fly it and see if I want to mess with it or just leave well enough alone.
Now that I’ve experimented and I know how I want to do this, I should be able to do the next one in about an hour. I have six or seven of the fine NVA assemblies on hand for further experiments. I find being able to do this with Medallions as well as TeeDees is quite nice. Also, next I am going to try this on a Medallion .09 I have on a White Lightning. Maybe this Thursday or Friday.
Mark