downunder,
I agree, the length of the glowplug shouldn't matter, and I was really surprised to see this OS .46FX go from not running well to running perfectly with just a change in plug like that. As far as I know, the two plugs are roughly in the same heat range. Anyway, it fixed it, much to my surprise.

SpaceClam,
Actually, you have that backwards in an ABC engine. In ABC engines, the liner expands faster than the piston, that's why a good ABC engine binds at TDC when cold, but doesn't grind itself to death when running. The running temp causes the fit to loosen a little. This is also why it's bad to run an ABC engine slobbering rich or even at idle for a long period of time with out getting it warmed up first. Cranking away with a starter is bad too. Most ABC engines, if you over heat them, just loose power and arn't harmed. They don't seize, usually (It's possible to get varnish buildup on the sides of the piston that can then jam things up. That varnish gets there because of the overly loose fit of the piston-liner when over-heated).

What you are thinking of is ringed engines, where the ring expands faster than anything with heat, increaseing compression. An overly lean run can easily cause the engine to seize up and create a paperweight. Iv'e seen that happen.

Also, our model engines do not actually require backpressure to run. Engines were run for many many years with no muffler at all and they ran just fine. I've had mufflers fall off in flight, and the engine runs fine. LOUDER, prodcues more power, but otherwise fine. It is true that some engines draw fuel poorly with out muffler pressure to the tank, though I doubt this is the case here.

Highlander,
It sounds like you have a lot of experience with a lot of different engines, and moreover, it sounds like if you are with out an engine for 2 weeks while a service department does it's thing, you have other planes to fly. I agree that if an engine is actually faulty it should be sent back, and it sounds like you'd be a good judge of when to send it back or not. But for a beginner, my guess would be it's not set properly first, and faulty second. So before he ships it back, I'd suggest getting another set of eyeballs on the problem, and if it still won't run, then it's worthwhile considering sending it back. And yes, I've seen some really funny "bad" engines. One engine a friend of mine had, the engine mounting holes weren't drilled. On another, the muffler outlet wasn't drilled out.

On RNV assemblies, on my OS .40LA and .25FXs, I don't have any problems. As someone else suggested, try changing the fuel line between the RNV and carb. It's realy easy to get a pin hole in there. I holed one of mine while mounting the engine. Also check to make sure the bits are all firmly screwed together, and seal the needle with a bit of fuel tubing.

When I have a choice, I prefer the conventional needle valve, and I like carbs like the Magnums that allow you to convert it back and forth between the two arrangements. IMHO, that's what OS should have done, given people the option.