I've been looking at how to convert the N Gauge Society's Hunslet shunter to 2mm finescale for quite a while.
Superficially it looked easy - same bearings in split frame as Bachmann have used in several of their smaller steam shunters, such as the Jinty and 57xx. But, there were a few "gotchas" to catch those who might rush in. The main one being the very tight clearance between a spur gear in the mechanism and one of the axles. That meant the usual 2mm finescale parts for the Jinty conversion wouldn't fit without modification. The muff connecting one axle needs to be thinned quite considerably, to less than 2mm O/D (when its got a 1.5mm hole inside it, that means a very thin wall). Its possible if you have a lathe, and might be possible with care by holding a muff in a minidrill and filing/sanding the muff down to size (a sliding fit axle inside the muff stops it deforming). In the photo below, the left side of the muff has been reduced from the nominal 2.3mm diameter of the right side:
With that step out of the way, the rest is "normal conversion" - coupling rods to make (or reduce the hole size in the N gauge rods), and drop in the wheels.
The other approach is to finescale the supplied wheels. This is relatively simple to do for the lathe owner, the wheels are brass, so machine easily. Reduce the flange height from 0.7mm to 0.5mm, and shave about 0.4mm off the back of the wheel, then gently round the edge of the flange with a file whilst the wheel spins in the lathe. The hardeset part of this job is work-holding: I made a tool which centred the wheel precisely, provided a drive pin, and had a shaft through the axle hole to a clamping domed washer and nut (sounds more complex than it really is!).
The N wheels have a back-to-back of around 7.3mm. For 2mm finescale it needs to be 8.5mm, an increase of 1.2mm. The re-profiling gets 0.4mm per wheel, so 0.8mm, but a further 0.4mm is needed by sliding the wheels out on their axles. This is OK, but I felt it was better if the bushes were replaced with slightly wider ones to reduce the amount of axle side-to-side float: for which the conversion bushes to use 2mm Scale Association wheels are suitable if drilled out to 3.0mm. (I made my own, but that was because I hadn't got any conversion ones to hand and have enough machinery to make them quickly).
So, neither method is quite "drop-in", but both fairly simple.
Below is some video on YouTube of one loco running through a 2mm turnout. This had the wheels turned down. I'll get round to uploading some video of one using 2mm wheels later.
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