Bachmann K-27

Bachmann K-27

one of these Bachmann’s K-27’s has been floating around for a little while – and after a point ended up on my doorstep, to try and fix-it, reporting as wobbling and occasionally stopping. The Bachmann locomotives are highly detailed Fn3 Scale (1.20.3) locomotives faithfully modeled after the Denver and Rio Grande Railroad, which is today a Narrow-gauge railroad. In the modeling world 1:20.3 scale, calculates out, and is considered a true scale for running on 45mm gauge track, the math shows us that within the 1:20.3 scale that 45mm gauge track is almost exactly to scale as 3-foot narrow gauge track.

because this, many of the modelers that work with this scale, are concerned with details and ensuring that they are correct, they also demand this from the manufactures.

Bachmann has tried to do this with the spectrum line, that is also scaled as Fn3.

Here’s a brief note about F scale – F scale has been around now for a few years, and is recognized by the National Model Railroad Association – it’s scale is 1:20.3, F scale standard gauge rolling stock and locomotives run on 70.64mm track, in 1:1 measurements this is 4’-8.5” as compared to 3’ narrow gauge track, which is the same gauge track used for 1 Scale to G scale to say what scale and gauge track we model in, we simply say the Scale, in our case F, then narrow gauge, at 3’ the same rule applies to HO, and O scales – for example you may have HOn3 and HO, or even O and On30.

dual gauge track, and a Bachmann Fn3 sitting on a section of duel gauge track.

So, why is this important, Scale is everything when building a layout and the level of details. The K27, a Mikado class locomotive with 27,000 lbf of tractive effort were basically a Standard gauge locomotive that was re-gauge as a Narrow gauge locomotive, additionally they have outside counter weights, that were as wide as the standard gauge, this means these are some of the largest Narrow gauge engines, and could not go where some of the small narrow gauge geared locomotives could.  – it also means that if you are building you layout for 1 scale (1:32) or G scale (1.22.5) and you have bridges or tunnels that are for those scales you will not be able to successfully run a Fn3 K27 locomotive on your layout.

This is a problem for the 1:1 locomotive as well, for example the D&RGW that uses K-27 locomotives is 3’ narrow gauge track, as is Roaring camp, which runs Narrow gauge geared locomotives such as Shays and a Heisler, there are parts where they’ve cut into the trees to make clearance for the locomotives, and other parts where they run into branches, they carry a chainsaw in on the tender of the locomotives.  – a K-27 couldn’t run on the roaring camp railroad because of its physical width.

Bachmann’s K-27 is a faithful reproduction, the locomotive has working suspension, and with the side rods, that pivot points allowing them to work with the suspension. The side-rods are attached to the outside counterweights, with bearings. The wheels themselves are mounted on a sleeve over the main axle – with bearings and springs to keep everything aligned the counterweights are then mounted on this center axel and quartered correctly.  The wheels themselves easily rotate on the main axel by about an 8th of a turn. Additionally, the counterweights have some slop in them they can move maybe up-to half of that 8th by themselves.

this, in theory will allow of a very smooth-running locomotive, this is if everything is correctly weighted, balanced and doesn’t have too much slop. Here’s the key word too much. What’s too much? After the model has been run a for a while parts start wearing out and developing enough slop to cause the rear counterweights to go out of quarter, this is first noticeable when the locomotive starts wobbling. It can get so bad that it will lock the side-rod and prevent the locomotive from moving.


Note the pivot point on the main side rod, this is on the last two drive wheels of the locomotive

So, the question becomes how to fix it – we can replace the counterweights, to decrease some of the slop, if you notice the rear counterweight isn’t exactly inline with the other counterweight, but after time this problem will develop again.


Now, I’m sure this design, works great for the real locomotives, however, the physics of how the side-rods work is different, on the real locomotive, they are pushed externally from the piston in the front, via the connecting rod, which is attached outside the of the main-rod on the second to last drive wheels, just before the pivot on the main-rod.  It’s said that when that when the weights are being re-balanced that they need to be careful to stay within the width of standard gauge track.  these counterweights work to help keep the push from that rod balanced. So that it rotates smoothly. basic mechanical physics.

So, the counter of this, is now rather than external force, the models motor drives the same axel/wheel configuration as the real locomotive, but internally.

So with that in mind, we have to look at the slop or movement between the wheels, side rods etc. and to make sure that are all aligned, and quartered, this means that each counter weight is aligned and they are one complete quarter off from the opposite side. If they move, they start causing the locomotive to wobble. As the wheels move out of sync.

so, back to the question, how do we fix this, without redesigning and rebuilding the axels, or putting a set counterweight and hoping it’ll stay. – lets take a different approach, lets look at how LGB address this problem for their Mikado.


Well, to start, LGB’s Mikado doesn’t have outside counterweights, for LGB locomotives that do have outside counterweights, they attach to the main axel the same as Bachmann, however the wheels are also pushed onto that main axel. There is no movement.


There’s a pivot point, but now it’s moved forward onto the wheel, with a fake one showing, there’s also room for the main side rod to move back and forth, with the two center drive wheels being suspended, while the front and rear wheels do not have any suspension. The other hidden trick LGB did, is that the drive wheels are articulated, so the main side-rod must move with that articulation.


That kills the idea of using a or copying design aspects of the side-rod from LGB.

Back to the side-rod, lets just set it up so it’s fixed, without a pivot point, sadly this would cancel the independent suspension aspect, and well with the counterweights, it could still lock the wheels, while increasing the wobble.

Ok, this problem was driving me crazy, these are different thoughts, and investigations I did, I compared 3 different Bachmann K-27s, Talked to a few different people about this problem.

Bachmann trains, has no solution, without simply replacing parts. – and sadly, this locomotive had parts replaced.

Here’s the funny part, the solution without extensive redesign was the easiest, all I did was take one side rod, and turn it around. Moving rear pivot point to the front. This offsets the pivot, preventing the combo of the side-rod and rear suspension from pivoting out of quarter, and locking the wheels.


To continue the story, this hasn’t been tested on a layout with uneven track, and truly the next step would be machining set screws, in the counterweights, as well as possibly making new parts with greater tolerances








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