Convert Marx 7-inch to 8 wheels

There’s a lot less interest in Marx’s short-lived 7-inch line, than in their other lines, but they can be useful cars. For example, with the appropriate trucks under them, they can add some variety to Marx’s metal scale cars. While they’re not quite 1:64 scale in most cases, they’re close enough that they look fine with them. Here’s how to convert Marx 7-inch to 8 wheels.

The Marx 7-inch cars, rushed to market in the late 1940s to compete with Unique Art, are something of an awkward middle child in the Marx product line. The lithography on them is usually attractive, and includes roadnames and designs that are missing in some of the other lines. But the 4-wheel design makes them look cheap. They look nicer with 8-wheel trucks on them, giving you a bit more variety when you want to run them with other types. And the size blends in well with the 3/16 line.

Removing the wheels, axles and couplers

convert Marx 7-inch cars to 8 wheels
Marx 8-wheel trucks fit perfectly on the 7-inch cars, and the size is close enough to the Marx 3/16 cars that they blend together well.

I generally don’t convert pristine cars. But I don’t feel guilty about converting 7-inch cars that are in less than pristine condition. It’s easy to swap in an unconverted frame to convert a car back. If the frame is in nice shape and I have a frame that’s less pristine, I’ll set aside the original frame and convert the beat-up frame.

To convert, remove the wheels and axles. Next, drill out the brass eyelets holding the couplers and remove the couplers. You need a drill bit that’s just barely too big to fit inside the eyelet. A 7/64″ bit works well. A 3mm bit would probably also work well if you have metric bits. Drill slowly. You only need to drill far enough to release the eyelet.

If you have an open car, the rest of the conversion is easy. If you have a car with a roof, such as a boxcar or caboose, you will need to remove the body from the frame. Look for tabs on the underside. Slowly and carefully straighten them just enough that you can lift the body from the frame. The less you bend the tabs, the less likely they are to break off.

Converting the Marx 7-inch frame to 8 wheels

Convert Marx 7-inch to 8 wheels
If you look closely near the center of the photo, you can see where I broke off the fake truck side to attach the replacement 3/16 trucks on this beat-up caboose.

The next order of business is to remove the fake truck sides that held the original axles in. It’s difficult to get tin snips in close enough to cut them, so what I do is bend the fake sides back and forth with a pair of pliers until metal fatigue sets in and they snap off. If you have an awl and a ruler and a steady hand, it helps to scribe a line along the edge where you want it to break. It helps ensure you get a straight, clean break. But the metal will tend to break off fairly straight regardless.

A dozen or so back-and-forths usually does it. Then file what remains smooth with the rest of the frame.

Attaching replacement trucks

convert Marx 7-inch to 8 wheels
I attach trucks with a machine screw and a stop nut. Also be very careful when bending tabs, and after reassembly, twist the tabs rather than folding them over when you can.

Use the holes where the couplers were mounted to attach trucks. They line up perfectly, which was by design. I use a 6-32 machine screw and a stop nut. This gives me more control over the tightness, and makes it very easy to change trucks if I ever want to. I like the use 3/16 trucks, but you can use the later trucks and run them with the later plastic cars too.

Mix and match trucks and couplers as you wish. Nothing stops you from putting prewar trucks on one and running it with 1920s and 1930s cars of any make either. It will look the part, and the trucks will probably cost more than the car in most cases.

When you reassemble the car, insert the tabs from the body into the slots on the frame like they were before. But rather than folding the tabs back over, just twist the tabs slightly. The car isn’t going to get the kind of rough handling Marx anticipated in the 1950s, so twisting the tabs is enough to hold the car together today. And that puts less stress on the tabs, so if you ever need to disassemble the car again, you can do so while minimizing the additional stress you put on the tabs.

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