I know a lot of 3RS modelers retain the original coil couplers. Converting over to Kadee's isn't that hard. Helps to have a drill press but you can get by without one. You will need taps and small bits as well as some brass stock. A good soldering iron or a small butane powered one. The big issue with tenders is the fact that where you want to place the coupler is right where the frame and tender body overlap.
Start buy purchasing some brass stock. Place the coupler lid on the end of the brass strip. Nice and centered. Mark the hole with a center punch.
Drill and tap for a 2 56 screw. Mount the assembled coupler and drill out the rear hole. Don't tap it just yet. Cut the brass stock just beyond the rear hole. Now fit a piece of stock to form a T pattern. It should look like this when your done. Constantly check your work as far as how it fits in the tender. Square it up and solder the two together. Hit any excess solder with a file. It should look like this minus a few holes.
Now finish drilling and tapping the second hole. I drill out the underside piece so that I am only tapping the coupler mount. Now lay the piece on tender floor. I use some brass stock to space the mount from the tender body. File notches if needed to go around protruding screws if needed. Just make sure it sits flush on the frame.
Check the fit as far as protruding screws and speaker placement. Find a spot to drill 2 holes. Make sure the coupler is centered in the body. Either use a reference point and eyeball it or a ruler. I tapped the brass for some 3mm screws I had. You can also do the tender floor or through bolt it. If it's slightly off. Don't panic. Some needle files can elongate the holes to shift it over.
One thing I did have to modify was the tender truck. The mount for the original coil coupler contacted the new coupler mount. Some quick work with a file took enough material off for clearance and still left plenty of meat left if you decided go back. Bring it over to the height gauge and you will need some shims. I used 3 from Micromark with some 3/8 screws to secure it.
This is what it should look like. The final pick shows it painted and weathered. Ready to go to work picking up and setting out cars.
This method works on just about every steamer. As far as the front coupler. Pretty straightforward. The hard part is fishing out the factory coil coupler. When I get it done I'll post some pics.
