A few threads asked about how to convert the MTH F7B dummt to three rail. so I thought it warranted a separate post:
First a little background: MTH has just recently released a set of scale F7 ABA units with both scale and high rail wheels. Road names include PRR, Southern, Maryland Midland and five others. While the ABA set can be ordered with scale wheels, the addditional, seperate purchase B unit dummy is only available with high rail wheels. Here is what I did to convert the F7B 3 rail dummy to 2 rail.
Wheelsets: This is easy. These are PS3-2 locos, so the 2 rail conversion axles (20-89005) drop right in. It takes less than 5 minutes. Right now Trainworld has them available and at a very good price
Couplers: This is a bit more involved. The 2 rail couplers are mounted to the trucks. Mounting the Kadee in the conventional position (on the chassis) and still allow easy removal of the shell, is more work than I wanted to do right now. I plan more extensive mods to these later, but for the moment I just wanted to get them on the track. Here is how I did it:
1) Remove the shell (four screws at the corner of the chassis) and remove the truck. (One central screw).
2) Remove the three rail coupler where it meets the truck block. (one 2mm diameter x 4 mm long screw)
3) Bend a piece of 0.064 x 2.50” brass to a dog leg shape so it fits into the slot where the three rail coupler did, and puts the kadee at the right position (both height and stick out). Use a cardboard template to get it right first.
4) You will have to narrow the bar (the .250" dimension) where it goes into the truck block, as the width of the slot in the truck block is slightly less than .250.” File down the width so it slides in and out easily, but avoid too much slop.
5) Drill a #43 drill (0.890” dia) hole in the bar to match up with the threaded hole on the truck.
6) Drill and tap for two 2-56 holes to mount the kadee.
7) Assemble. For the bar to the truck, use a longer 2 mm diameter screw (6 mm long, vs the supplied 4 mm)
Note I would expect this process would be easier on MTH models of more modern diesels, as the 2 rail coupler should mount to the body in a more conventional manner
These pictures may help:
Here they are. Apologies for showing them in the same background I used for the Congo cars.
