I've had a few inquiry's on how I reduced the flange diameter on the new Athearn challenger, so I thought I'd post it up here for anyone who is interested in doing it themselves.
The process is really easy. Remove the retaining plate under the tender (3 screws plus coupler screw), all the tender wheels will then be exposed.
I removed one axle set at a time and removed one wheel from the axle by twisting back and forth.
Using a dremel I inserted the axle with wheel into the dremels chuck. I set the dremel at min speed and gently placed a small flat needle file on the outer circumference of the flange. File the flange down by about .5mm and round it off.
I then used some 1000grit sand paper to smooth it out.
Leaving the axle in the dremel, I removed the newly modified wheel and replaced it with the other one. I then repeated the above procedure for each wheel set.
When you are ready to replace the tender cover plate, ensure that the small electrical wipers are inserted behind the wheels. Also be careful that you don’t loose the small bearings.
The loco trailing truck’s wheel sets just pop out of the frame with no real effort. The leading truck on the tender is the same. The leading truck of the loco has two small screws that hold the axles in.
When grinding the flanges be careful that you don’t hit the wheel treads with the file. Patience is a must here.
I used the Mk1 eyeball but you could use a caliper to ensure that you only take the correct amount of material off of each flange.
Here's a pic of the newly modified wheels:
