Fine-tuning HO knuckle couplers
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Not long ago the only way to have working knuckle couplers on your HO cars was to install them yourself. The standard coupler on most cars straight from the box was the horn-hook. The only widely available knuckle coupler was the Kadee Magne-Matic. The biggest knock on the horn-hook was (and is) looks: They are ugly and bear little resemblance to a prototype coupler. Operationally they are also lacking: They pull just fine, but tend to skew sideways while pushing and are near impossible to uncouple automatically (and not easy to uncouple by hand). Their only advantage was that they were free - they came on virtually every locomotive and car. Kadee's no. 5 coupler thus became the de facto HO standard that modelers strove for. Although a bit oversize, the no. 5 looks like a real coupler. It operates very well, has the advantage of delayed magnetic uncoupling, and can be uncoupled by hand as well. Today, along with Kadee, other companies offer automatic knuckle couplers, including Accurail (Accumate), Bachmann (E-Z Mate), Life-Like (Proto 2000 coupler), and McHenry. For a comparison of the different brands, see Jim Hediger's article in the July 2000 issue of Model Railroader. Now both Kadee and Accurail also offer automatic knuckle couplers that are scale size. Both operate well, but must be mounted and adjusted properly to ensure good operation. Even though most equipment now comes with knuckle couplers as standard, many of these allegedly ready-to-run cars require some coupler tune-up for optimum performance. It's a good idea to check every new car before placing it in service. |
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Most model cars today have couplers mounted on the body of the car, which is the way the prototype does it. Many older models (and many cars designed for the train-set market) have truck-mounted couplers. Which is best? Pulling usually isn't a problem, but pushing truck-mounted couplers around curves causes the trucks to skew forcing the flanges against the insides of the railheads. This pressure often results in derailments, especially at rail joints or turnouts. Body-mounted couplers work better as they transfer these forces through the carbodies, so I save truck mounting for passenger cars and other long cars that have problems on tight curves. On many freight cars the coupler box covers are held in place by friction pins; on some others the manufacturers recommend gluing the covers in place. Neither method is fully satisfactory as friction pins can work loose over time and gluing boxes makes it difficult to correct any future problems. A better solution is to mount these boxes and box covers with screws. It's a simple and inexpensive process to improve reliability. Start by cutting off the friction pin from the coupler box cover. Use a scriber to press a dimple in the center of the former friction pin location and drill a hole through it with a no. 43 (clearance) bit. Check the size of the hole on the body with the no. 50 bit. If the hole is smaller, drill it out with the bit; if the hole is larger fill it with body putty, then when it dries drill it with the no. 50 bit. Tap the hole 2-56. The tap will easily cut threads in plastic and soft metal. Turn the tap in one revolution, then back it out to clear scrap material. Repeat the process until the threads are through the hole. Use a 2-56 roundhead screw to secure the cover. |
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Preparation and installation To install and maintain couplers you'll need nos. 43 and 50 drill bits, a 2-56 tap (for 2-56 screws), a pin vise to hold them, and a hobby knife. The no. 50 bit is for the 2-56 screw mounting hole, and the no. 43 bit is the proper size for drilling clearance holes. You'll also need what I consider an absolute necessity, regardless of the brand of couplers you're using: a Kadee no. 205 height gauge. Checking every coupler on every car against the gauge before placing it in service will go a long way toward ensuring smooth operation. I also highly recommend getting Kadee sample pack no. 13, which includes 25 different Kadee couplers. Regardless of the brand of coupler you're using, this sample pack is an excellent way to test various combinations of shank length, coupler boxes, or coupler head overset or underset. Before adding a coupler, inspect all of the parts. Check the inside of the coupler box as well as the coupler box cover for any stray molding flash, as this can keep the spring and coupler from seating properly. Clean up any problem areas with a hobby knife or needle files. Check the coupler shank for burrs. Any imperfections can keep the coupler from centering properly. Check the coupler knuckles for burrs and flash. The coupler shank should move freely from side to side without binding. If it doesn't, make sure the screw isn't on too tight. If that's not the problem, remove the cover and check for flash. If you're mounting a coupler using the draft-gear box that came with it, make sure you have a flat area on which to mount it. Use the clearance hole on the box as a guide to drill the mounting hole and secure the box with a screw. Once the coupler and box cover are installed and operating smoothly, add the trucks and check the coupler height with the Kadee gauge. |
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There are a couple of ways to fix a coupler that is too low. If the height difference is slight (no more than .015") the simplest solution is to add a washer between the bolster and truck. Kadee makes fiber washers in .010" (no. 209) and .015" (208) thicknesses. Avoid using multiple (or thicker) washers, as the car will be less stable, and while the coupler height might be corrected the car itself will likely be standing too tall. Extremely low coupler height problems may be resolved by substituting an underset-shank coupler such as the Kadee no. 21, 24, or 27 or the McHenry 15, 16, or 17. These have the knuckle set higher on the shank than a standard coupler, which has a centered shank. If the coupler comes out too high, use an overset-shank coupler such as the Kadee no. 22, 25, or 29 or McHenry 12, 13, or 14. Another common (and often misdiagnosed) problem is a sagging coupler. At first glance it looks like the problem is a low coupler, but a closer inspection reveals that height isn't the problem - the coupler is sagging. This ready-to-run Athearn car was equipped at the factory with McHenry couplers that have shanks quite a bit thinner than the depth of the coupler box, allowing it to flop up and down. The extreme curve of the uncoupling pin was no doubt to keep it from snagging the track. Once again, there are a couple of ways to fix the problem. Most coupler boxes on new cars today are made to hold the Kadee no. 5 and similar couplers. Substituting a Kadee no. 58 coupler in the McHenry's place eliminates most of this excessive play. I have nothing against McHenry - it's just that the Kadee coupler has a slightly thicker shank and the added thickness of the bronze coupler spring just makes a better fit on this car. This new coupler is level and matches the height gauge (always look at the top of the coupler when checking the gauge). The second solution, if you want to save the original coupler, is to add a shim inside the coupler box to shorten the depth of the opening. When you change the height of the coupler you sometimes have to adjust its uncoupling pin. It should just clear the lower lip on the gauge. The easiest way is with a pair of uncoupling pin pliers such as the Kadee no. 237. A standard pliers will work as well, but the curved nose of the Kadee pliers makes bending the pin either way easier. |
| Be strict in testing There's no way a brief article can cover every possible coupler situation. Successful coupler installation is a matter of tight quality control and being creative in resolving tricky situations. If you take care to mount the couplers level, at the proper height, and functioning properly, you'll enjoy operating your layout more than ever. This article first appeared in the May 2002 issue of Model Railroader. |









