Holding It Up
I use #20 galvanized wire in 5-inch lengths, and pre-bend one 60-degree angle ¼-inch from one end. By some quirk, if you tightly wire the uncoupler assembly into the lower track mounting plate, you get a top-of-the-field-plates height equal to the height of the top of the rails—which is exactly what you want. Make sure the lower track mounting plate has its low part facing downwards and then insert the wire into the mounting screw's hole all the way until it hits your bend. Put the coil-and-plate assembly in place up through the mounting plate until it's tight into the plate slot. Bend the wire around the bottom of the coil, against the center of the wire coils—keeping the coil's end wires off to one side—and insert it up through the other mounting screw's hole, holding the other end of the wire tightly as well. Bend the end inwards to make another 60-degree bend. Now use a pliers to compress the wire at each end and use a rail nipper tool to trim this wire to ¼-inch long. If it's too tight the lower track mounting plate will be bowed; if too loose, the assembly can be moved a bit in its frame—it should be tight and only sideward tilt should be possible. The tilt will be adjusted by CA glue—more later. Looseness is to be avoided because it will prevent you from getting the field plate tops as high as the rail tops, so it won't work right.
Once the assembly is together, put it the roadbed in place and the assembly onto the subroadbed hole, then put the tie-less part of the flextrack over the assembly (see diagram 5) and fasten it to the roadbed with two nails—each as close to the uncoupler hole as possible. The rail and plate heights should be the same. Now put two cars with bent trip pins on the track and couple them up. Next use alligator clips to attain a powered uncoupler (for no longer than three seconds at a time) and test the assembly. You can tell from the symptoms if the uncouplers are too high (trip pins catch on them when trains are running), too low (the magnetic force on the trip pins is either too weak for any uncoupling or too weak for delayed uncoupling), not parallel to the rails (the uncoupler has spots that work well and other spots that don't), not centered between the rails—or tilted from side to side (one of two couplers moves a lot when activated but the other moves barely at all), or tilted from end to end (low end won't work and high end works great but passing pins run into it). Note: sometimes after plate bending you find that the top of one plate is a bit higher than the other when the assembly is vertical, so you either have to loosen the screw and tap the plate higher or mount the assembly a bit tilted in the mounting plate in order to get the tops of the plates at the same height between the rails. While you're at it, make sure the two plate tops are precisely parallel to each other (fine-tune your bend if they're not) and to the rails (swivel the assembly in the loose subroadbed hole if they're not).
Once you've gotten them to work, if they're anything short of perfect, remove the nails and track and do a final alignment by use of a 6" or 7" hobbyist's ruler. (If everything works perfectly, skip to the next paragraph.) Balance the center (the 3" mark for the 6" ruler, the 3.5" mark for the 7" ruler) of the ruler on the field plate tops. The ends of the ruler should be equal distances above the benchwork surface. If the surface isn't level, get a small, short level as a replacement for the ruler—get the top of the field plates level, even if you have to tilt the field plates a little in the lower track mounting plate. Now turn the ruler/level 90 degrees and level it again.
Once it's perfect, put a 1" bead of thick CA glue along the intersection of each field plate and the lower track mounting plate, without disturbing the alignment. (See diagram 4.) Wait a day before you disturb the uncoupler. When you permanently install the assembly, use a drop of CA under each of the four lower track mounting tabs keeps the assembly from slipping on the plywood, but nothing but wire and the mounting plate is needed to hold the uncoupler up (see diagrams 4, 5 and 6).