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USA Trains SD 40-2

The SD 40-2 is an impressively long engine with an overhang that is just as incredible. What is overhang? Well, it's what happens when an engine this long runs through unrealistically tight curves or turns. The ends of the engine swing way out to the outside of the curve. Our layout has a minimum curve of 4 feet in radius (8 foot diameter) which is what USA recommends as the minimum to run this engine on... Having said that, seeing this engine run around our layout and go through the curves shows that the couplers actually hang out past the outside rail of the track. Basically that means the couplers need to swing from side to side in an extreme way or else the trailing cars will most likely get pulled off the track by the engine. Not a good thing unless your Gomez from the Adams Family... To get around this problem USA supplies a knuckle coupler that runs in a track allowing it to slide from one side of the engine all the way to the other and a couple of springs in the track also serve as a nice centering device. When I got my engine one of the first things done was to remove the hook and loop couplers and install the wide swinging knuckle couplers. After a few tests I came to the conclusion that even with the wide sliding ability of the couplers, it was just barely enough to make everything work without any problems. Now, I'm changing this engine over to Kadee couplers and I hope they will be able to navigate my turns and switches without any problems. Kadee recommends two part numbers for the conversion, either the 831 or the 837. I originally ordered the 831 thinking it would be fine but after getting the coupler put together and mocked up to the engine I realized that the coupler was going to hang out way too far for my taste. So I ordered the 837 and found it to be just perfect. The coupler gear box hides under the engines frame and the coupler just hangs out from the engine enough to look like the real thing.

Here you can see the stock USA hook and loop coupler in the upper right corner of the picture. The USA wide swinging knuckle coupler is in the upper left corner and the new Kadee 837 is at the bottom (note the screw in the 837 is only there to hold the coupler together for the photo). Along with the parts from the 837 kit we need the mounting bracket and post from the hook and loop coupler and two # 4-40 x 1/2" long screws that aren't supplied with the kit. I bought a bag of them at Home Depot for under a dollar.

Before we could mount the new coupler to the old hook and loop bracket a small modification had to be made. Kadee tells you to shorten the bracket by 5/8" or trim it very closely to the coupler mounting hole. I did that on both of my brackets and later realized that leaving a tiny bit more on the bracket might have made it a little stronger although mine seems fine this way. Pictured here is a stock bracket on the left and a modified bracket on the right. If you trim a little close like I did there's no need to worry, you'll see in a couple of minutes...

The next step in the conversion is to test fit the empty coupler gear box on the USA mounting bracket. All you have to do is hold the gear box up to the bracket and look through the mounting hole at the front of the box to be sure the holes line up. If not, there is a lip on the top side of the gear box that Kadee wants you to trim to make room for the box to slide back on the mounting bracket allowing the holes to line up. Mine needed to be trimmed but instead of trimming the lip on the gear box, I trimmed the back edge of the mounting bracket. That way I had the lip to give a little extra strength if the thin screw hole wasn't enough. Once the trimming was done I assembled the coupler without the gear box cover and gently screwed the assembly to the bracket mount with one of the # 4-40 x 1/2" screws.

The last step was to simply attach the entire mount assembly to the engine with the original screws and centering spring.

That's pretty much it for the conversion, a quick check with the coupler height gauge showed everything to be just right.

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This site has been online since: Tuesday July 28, 1998 and was last modified: Wednesday July 28, 2004.