New Axles for a Second Generation Bachmann HO 4-8-4 Northern – Part 3

Happy New Year!

For the first post of the New Year, I thought I’d start by releasing a new product that would also finish a project started last year.

The second generation of the Bachmann HO 4-8-4 locomotives, just like the first generation, has an issue with splitting axles so I’ve designed a set of 3D printed replacements. You can read the first post about this here and the second here.

The second generation 4-8-4 chassis is an improvement on the first and has a much thicker drive gear, which in turn means it has stronger teeth. This means the 3D printed gear axle can be a direct copy of the original, although I’ve made some very minor changes to the overall design.

The replacement axle set will contain three axles and one gear. As you can see below the axles all have square holes which makes quartering (setting up the valve gear and side rods) much easier, because each wheel can only fit at 90° rotations. One thing that’s important is to clean out this square hole as it will almost certainly have some 3D print residue inside. Although this is a waxy substance it still has a thickness and if the wheel is inserted before removing this, the fit will be too tight and the replacement axle may crack. I use a very small flat blade screwdriver, or needle file with a square point to run along the square hole corner and scrape out the residue.

If the original gears have split and no attempt has been made to fix this then the old parts can simply be pulled out, they may even fall out, and the new parts can be put in. But if an attempt to repair the original axles with glue has been made then this will need to be cleaned up. On the model below the rear two axles came right out but the front two have been glued. Using a pair of side cutters I cut or rather cracked the axle in half.

Some parts then fell away but some remained stuck, so I used some pliers to pull them out. When doing this be mindful of what you’re holding onto and how the force of pulling is restrained, as you don’t want to bend and mangle the side rods and valve gear.

With all the plastic removed the void in the wheel will probably still have glue in it as you can see below. This will also have to be removed. How you do this will depend on the type of glue used, but the wheel is made from metal with a hard plated surface so normally it can be scraped off. I tend to use a mixture of a small flat screwdriver, craft knife and tweezers. I tend not to use files or anything abrasive that can damage the wheel.

To see if all the glue has been removed one trick I discovered is to use one of the old axles, if you still have one intact that is, to test and see if it fits. It doesn’t matter if the old axle is cracking, as long as you can fit it into the wheel. Below you can see the axle fitted onto a wheel that wasn’t glued. There’s a small gap between the wheel and the step on the axle which is correct.

Fitting the old cracked axle into the wheel with the glue, you can see it won’t go in as far, because there’s still some glue in the base of the wheel void. So this’ll need to be removed.

Once all the glue is removed the wheel void should look something like this. All the metal surfaces I scraped with the tools are now shiny, any glue even if transparent would show up as a dull area.

With all the wheel voids cleaned up and a test fit done with an old axle, the new axles and gear can be fitted.

With the wheels pushed all the way in, this should leave that small gap between the wheel and step on the axle; the wheels should be at the right spacing. But it’s always a good idea to check this with a wheel gauge. For HO and OO gauges 14.4mm (0.5669″) is the correct spacing from the back of each wheel, commonly known as the ‘back-to-back’. If the gauge is too tight a wheel can be pulled out ever-so-slightly. Be careful not to twist the wheel when doing this as you can crack the axle. If the gauge is loose then one of the wheels need to be pushed in further.

And that’s it, the base plate can be refitted and the locomotive is ready to go.

My replacement axle and gear kit for the Bachmann HO 4-8-4 Second Generation is available here.

Next week I intend to share an update for the HO Baldwin RT-624 project with you.