Modifying Sergent Couplers to Fit Kadee #78 Pockets

In addition to using PROTO:87 wheels on the Rail Yard Models Penn Central X58 boxcar, I am substituting the Kadee #78 with Sergent scale couplers. The Sergent couplers and the coupler pockets need to be modified slightly to work together. It’s quite easy but takes little time.

Required tools and materials

  • Sergent EC87A assembled die cast AAR type E couplers with compatible shank*
  • Sergent AFC precision cast EC87K assembly fixture
  • needle file
  • X-Acto knife
  • fine sand paper
  • digital caliper

Modifying the coupler

Start by putting a Sergent coupler in the assembly fixture. I found that using the fixture minimizes side to side movement and makes working on the coupler much easier.

Take a needle file and start thinning the shaft. Turn the coupler over on occasion to make sure that an equal amount of material is taken away from both top and bottom.

Unmodified Sergent coupler in assembly fixture.

Unmodified Sergent coupler in assembly fixture. (click to enlarge)

Sergent recommends to thin the coupler shank to 0.057″ (1.45 mm). Use a digital caliper to check the thickness from time to time. The modified coupler should look like this:

Sergent coupler with thinned shaft.

Sergent coupler with thinned shaft.

Fitting the coupler in the pocket

To fit the coupler in the Kadee #78 pocket, Sergent recommends to widen the hole to 0.135″ (3.43 mm) with a tapered reamer until it fits over the mounting post. I found that widening the hole is not necessary. Instead, I forced the coupler over the mounting post and subsequently chiseled off the sides so that the coupler is able to move freely on the post.

Modified Sergent coupler in open Kadee #78 pocket.

Modified Sergent coupler in the open X58 Kadee #78 pocket.

Next, fit the cover over the coupler pocket. You should be able to close the pocket without applying force. When the pocket is closed, the coupler should still move freely. Hold the pocket sideways and check if the coupler falls by sheer gravity. If it doesn’t, thin the shank some more.

Complete draft gear with Sergent coupler.

Complete draft gear with Sergent coupler.

As a last step, insert the spring that comes with couplers and fit it between the coupler and the mounting post. Note that the spring is not intended to center the coupler. It is only supposed to provide friction against side to side movement. Glue the pocket shut. Done!

If you have any questions, leave a comment below.

*If you prefer, you can use Sergent’s EC87K easy-to-assemble die cast AAR type E couplers with compatible shank instead.

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