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Peter's Model Railroading | The Layout | Equipment | PRR X26
Coupler Conversion


This page describes the various coupler installations I have done on this car. The first one shows how I converted this car from the toy-train couplers and wheels that the model comes with to the scale ones. It is an easy process. The S-Helper Service box comes complete with the scale wheels, but you will have to buy the scale couplers separately. I decided to purchase a set of S-Helper Service couplers (part number 01295 - provides four couplers). The only tools required for this project are a set of small jewelers screwdrivers, tweezers, and perhaps a magnifying visor. The first photo shows the car as it comes out of the box. It has the large American Flyer-compatible couplers and wheels with large flanges.

This is a close-up photo of the original wheels, truck, and coupler. The coupler is truck-mounted, to handle the tight curves that AF-based layouts typically have.

The first step is to remove the truck. I had a hard time with removing the screws. They are in there very tight. Be careful of the small washer in the truck, because it is easy to get lost. The wheels can be removed by slightly widening the truck. For one truck this was very difficult, for the other the wheels practically fell out. Once the first wheel is out, the other one is easy to remove.

These are the new parts. I was very impressed with S-Helper Service's fore-thought to us scale modelers. I was fully prepared to have to cut the shim off of the old coupler, but when I opened the package of the wheels, I discovered that they provide us with a replacement shim (seen here in the lower-left corner of the photo).

Before placing the truck back on the model, I installed the new S-Helper Service couplers. The car is already set up to take the screws for the coupler (or, alternatively the Kadee S-scale coupler). I found it easiest to install the two screws on the outside edges first. The center screw is seen on the car's floor, and the new truck shim is already in place in this photo.

Next, I installed the center screw. If you look closely at the photo, you may see that the center screw isn't all the way down. If you tighten it too much, the coupler will not move freely. Also, I found I needed to loosen the screw just a bit more so that the coupler would return back to its center position.

And here is the finished model. The truck fits nicely in between the pins on the shim, and is then attached with the screw. I usually tighten the screw all the way and then back it off a half turn or so to allow the wheel some sideways motion. All-in-all, this was a simple project. The hardest part was loosening the trucks' screws. S-Helper Service provides us with all we need to easily convert these cars to scale. The visual difference between the toy couplers and the scale ones is significant. Since I don't plan on using uncoupling magnets, I will most likely cut off the trip pin on the coupler. However, that will be done later when I further detail this car. The scale wheels exactly matched the NASG S-scale gauge.

Walthers Proto MAX Couplers
For a good number of years, these oversized HO-scale couplers were my standard for my fleet. This photo shows that coupler installed on this box car. Note that I used only the center screw. In a heavy crash, the coupler can move, but most of the time, it stays put. For some of my cars I have put a drop of superglue in the back of the coupler where it meets the floor of the car.

Inventive Models Brass Couplers
In 2022, Inventive Models announced their new brass couplers for S-scale. I ordered some and they arrived in mid-2023. Smoky Mountain Model Works makes coupler boxes for these couplers, which is what I used. This is a step-by-step guide on how I install these two to form the final coupler. The first parts to work with are the main coupler box part and its top lid.

I removed the truck from the box car (not easy to do with the S-Helper Service cars). I used the caliper to measure the distance from outside of the box car end to the truck bolster. It has to be the outside edge of the end of the box car, because the coupler box lid has a lip to it that needs to clear all body work (i.e. molded-in rivets on the car). Then, carefully lining the caliper up with the inside edge of the lid's lip, I marked off on the body where the box needs to be cut, so that it fully fits the car's body. As you can see, this was about 3/4 of an inch.

I used my Ultimation Slicer to cut the coupler box' end to length (an X-acto razor saw can be used also, but takes longer). Since the urethane-cast part is a bit brittle, it may not always yield a clean cut, but since this is hidden under the body and blocked from view by the wheels, it is not that big of a deal, unless you are going to take the car to a contest. I then gathered up all the parts needed for the installation. The Smoky Mountain Model Works parts need to have some flash removed, but that is easy to do by scraping an X-acto blade along the edges. The I.M. coupler requires no preparation.

This will be custom to each installation, but the S-Helper Service box cars have a metal weight glued to their interior bottom, right over the truck location (which is the correct thing to do). But, that means these long screws that come with the SMMW coupler box are too long. I kind of eye-ball their length and use a pair of cutter-pliers to cut off the ends of the screws. On one side of the coupler box, the location of where to drill the holes are marked off (see the photo above). So, I drill that with a drill bit sufficient for the screws. This then leaves the bottom-face side of the coupler box with the two holes you see in this photo.

I found that installing everything at once is a bit of a challenge, so I first do a test fitting by installing the coupler box with its lid onto the car using the modified screws. For the S-Helper Service box car, the holes lined up perfectly. By doing this test fit, the screws will cut the necessary grooves, and you can find out if you cut the screws too long or too short, and, if so, try again. The things to look out for here are, is the main coupler box too long to fit? (if so, file the ends down some more); does the coupler box sit relatively straight on the body (if not, the holes weren't drilled in the correct location, or the drill bit was inserted at an angle).

If everything checks out OK, it is time to install the coupler itself. There is a small tab at the end of the coupler box cavity, onto which the spring needs to be positioned. I did that first, and then I slid the coupler onto the tube. I then use a toothpick to gently compress the spring and get it to slip on to the back of the coupler, as I am pushing the coupler down. This has to be done with care, otherwise the spring may fly away. If found using a metal tool caused the spring to want to attach to it (some amount of magnetism or static electricity), hence the use of a toothpick. Make sure to orient the coupler head in the correct direction. This means the groove side of the coupler needs to be facing the bottom of the coupler box. In other words, this photo shows the coupler from top, down. By the way, I forgo the tiny spring on the coupler, as this coupler box does not allow the coupler to move in and out of the coupler box, so that tiny spring will do nothing.

It is then just a matter of placing the coupler top lid (I did not apply any glue) and carefully holding it in place while installing it with the two screws. I usually put the screws into the box a ways before installing the coupler and its spring, as it can be difficult to hold the coupler, the car, and try to get the screws started in their holes. I found that the screw in the back can be reasonably tight, but the one near the end of the car needs to not be overly tight, otherwise the coupler will bind. Test to make sure the coupler moves freely, and that it returns close to the center position.

When I re-installed the trucks, I did not install the shim mentioned above that comes with the SHS cars. When I set the completed car up on my coupler height gauge jig, the top of the coupler sits exactly where it has to be, i.e. the top of the wooden block of my gauge.