Assembly Instructions

Prototype Couplers

Assembly Instructions





Unlike Sergent Engineering, all Prototype Couplers couplers are investment cast. You'll notice that the investment castings aren't as clean those that come from the die casting process used for the Sergent Engineering EC87K and EC87N couplers. While I have access to Sergent Engineering Die-Cast equipment, I currently cannot afford to make an order large enough for production, Die casting also isn’t practical for all coupler styles because of the steel tooling used in that process would be prohibitively complex and expensive. Investment casting is a great solution for these couplers because the patterns can be arbitrarily complex. The downside is that investment casting is much more labor intensive and therefore more expensive from a part cost standpoint. The investment castings require more clean up and the dimensions of the castings can vary some from one part to the next.


What this means in terms of assembling couplers from investment cast parts is that you'll have to pay more attention detail than you do with Sergent Engineering’s die cast parts. This isn't a huge deal, but do expect to spend more time assembling and checking these couplers than you do with the Sergent Engineering EC87K and EN87K couplers.






Castings should be painted prior to assembly.









The first step in assembling the couplers is to ensure the castings are free of bubbles. Most couplers won't have these, but you should check just in case. Bubbles typically show up at the rear of the thin spot in the shank where the bottom cover goes. They can also be in the hole where the bottom cover pin goes. Furthermore they can show up in the hole for the ball in coupler head. These typically pop off when you attempt to cut them away with a hobby knife. An X acto #16 blade is preferred for this job verses the more common #11 blade because it is much stronger.


Next, drop a ball into the top casting to ensure that it goes in ok. If the ball doesn't drop completely, DON'T push it in because you might never get it back out. Instead, just turn the casting over and let the ball fall away. Clean the hole out by spinning a cut back toothpick point in the cylinder. Then try again. The sharp end of a hobby knife can usually be used to clean any debris from the hole. If you run into a casting defect where the ball just plain won't fall and nothing will help, then contact us for a replacement casting.


The bottom castings for these couplers are investment cast and are thin where the part is broken from the sprue. Sometimes the casting will bend a little as it is broken away from the sprue. You can straighten these parts by placing them into a top casting and pressing the front tip of the casting down with one thumb while holding the other end with your other thumb. This straightening procedure minimizes the gap in the shank and still allows the coupler to operate reliably. You can also use a small vise or modeling pliers to help close the gap further.


Assembly is pretty straightforward from this point. Just follow the same procedure you have become familiar with for the Sergent EC87K couplers.


Give the glue at least two hours to cure. Then it’s time to lubricate the couplers and break them in. I use an ordinary mechanical pencil (0.5mm) with standard HB (#2) lead. Be sure to get the inside surface of the knuckle and the ears on the side of the coupler. Couplers must be slid over themselves vertically as part of the break in process. You can't do that with two tightlock (type F/ H or Spear) couplers -- because they lock together vertically. A SEC87 coupler can be used for this process. You'll mate that coupler with the tightlock couplers to break in the tightlocks vertically.









Couplers are then tested by hanging an uncoupling wand off a desk lamp. Couple. Uncouple. Repeat. Go through this a few times until you are confident in the reliability of the couplers.


The most common problem with the couplers is that they will have a tendency to not lock. If you have a coupler with this problem, push the knuckle closed with your finger and watch to see if the ball falls. If so, then you must be pushing the knuckle further closed that it goes when mating to another coupler. Search for something that is holding the two couplers apart. Some possibilities are...


  • flash on the inner surface of the knuckle (repeat break-in procedure)
  • excess metal inside the "ear" on the side of the coupler (search and remove pry out bubble in the socket with a jeweler's screwdriver)
  • flash on the side point that fits in the "ear" (file point off slightly)

If the ball won't fall even when you push the knuckle closed with your finger, then the coupler will have to be disassembled to repair it. Things that can cause this sort of problem include...


  • no ball in the coupler
  • dust in the ball cylinder


If you need to disassemble a couple to fix it, soak the coupler in acetone to dissolve the adhesive then you can carefully pry the bottom from the shank. I do this by inserting a small flat head screwdriver into the front of the coupler where the knuckle moves and prying up. Then I start prying the bottom from the shank around the alignment pin.


The side view of the couplers can be improved on considerably by filling the joint in the shank with putty. Unlike Sergent Engineering’s couplers you do not need to fill the hole where the post from the bottom casting comes through since it has been eliminated.


A little paint touchup work and now the shank should look like a single massive hunk of metal!


The coupler is now ready to be installed. Be sure to adjust coupler height. That is very important with the tightlock style couplers.