Aristo-Craft GP40 & SD45 Loco Retrofit With Kadee 789 Centerset Couplers

Aristo GP40 & SD45 loco retrofit with Kadee 789 centerset couplers
Ted Doskaris
Aug 22, 2009
Revision GE-A1

May 18, 2018
Rev. GE-B  Alternative custom made metal coupler boxes

October 6, 2018 Rev. GE-B1 Removed moribund Aristo-Craft Link - Aristo is out of business 


The Kadee plastic coupler boxes described in this vignette (article) are modified (trimmed) in order to fit centerset type couplers on the Aristo SD 45 & GP40 locos without incurring modifications to the locos themselves.

Since this vignette was written, the modified plastic boxes were superceded (replaced) with custom made stronger CNC machined metal boxes.

The boxes utilize Kadee centerset couplers, springs & cover plate (lid) from a Kadee 907 or older 789 kit. The newer AAR "E" type coupler installed in the metal box is shown below.

The custom boxes were designed by me in concert with & made by Datum Precision, Inc. in Grass Valley, CA.

See vignette: "Custom Made Coupler Boxes & Kadee Centerset Couplers for Selected Aristo-Craft Locos"

That said, the information that follows is very helpful, particularly with respect to a method for removing the Aristo-Craft factory knuckle couplers so they are preserved without incurring irreparable damage to them.


Many of the Aristo-Craft GP40s I and other folks had acquired experienced coupler pull apart problems (not to be confused with unlatching) - even with moderate train loads. A fellow G scale modeler reports he resorted to using plastic tie raps as a solution to strap the couplers together!
It's profoundly unfortunate the Aristo knuckle coupler performance has retrograded with the first production run of these particular locos.
Given the time, money and effort to change out these couplers with Aristo replacements of like kind unknown performance quality, a better alternative is now borne out of necessity. Consequently, I decided to retrofit these problem locos with Kadee model 789s having centerset type couplers. In this article a Cotton Belt GP40 will serve as the primary example.
Though Kadee recommends using their model 787 having a medium offset coupler for both the GP40 and SD45, I believe it preferable to use the centerset coupler whenever possible - not only for its realistic appearance but for its better physics that is best suited for long, heavy trains.

Accordingly, this article will describe and show a method for removing the existing Aristo knuckle couplers, a method for modifications required of the Kadee 789 coupler box, the installation of the modified Kadee 789 assembly on the loco, and operational results.

The loco itself will not require any alterations as only the Kadee 789 coupler box is to have the necessary modifications.

The front and rear pilot areas of the GP40 loco are the same and are relevant with respect to the couplers in this discussion - so the material presented herein applies to both ends of the loco. Moreover, the Aristo-Craft GP40 and SD45 appear to share identical pilot assemblies and chassis mounting post designs, and as such, the modified Kadee 789 coupler assemblies can be used on the SD45, too. In this regard, the differences between the locos are meaningful due to the closer proximity of the SD45's larger 3 axle truck to the pilot that primarily affect maneuvering room in which to work

As of this writing, Aristo-Craft does not appear to have an exploded parts diagram of the GP40, but does have one for the SD45.
Examining the diagram will be helpful to see how the parts fit together, particularly of interest for the pilot inner & end sills, the chassis' coupler mounting post, and the Aristo knuckle coupler assembly.

Removing the Aristo knuckle coupler assembly

Though the pilot inner sill is secured to the underside of the loco's chassis by two screws, it is to be appreciated that the pilot end sills seem as though they are bonded to the ends of the chassis during the factory build process! Since I had no desire to remove the end railings, it's possible leaving them in place contributed to this effect.

Removing the pilot's two inner sill to chassis screws will allow some flexing to be had that can facilitate removal and installation of couplers, but be advised this is undesirable as back and forth flexing of the pilot assembly can be damaging.

For the description included in this article, those pilot screws were NOT REMOVED.

That said, the removal of the coupler assembly does not require it to be cut off as purported by some folks. Of consideration, too, is that the process of cutting or sawing it off in a confined space has risks of ancillary damage to the surrounding loco parts.
Instead, the coupler assembly can be dismantled in situ (left in place) in order to facilitate its removal - albeit in collective pieces.
The below picture shows what it looks like when taken apart. The long needle like item (with 90 degree bend) in the lower right corner is the coupler arm's wire type centering spring.

First remove the coupler centering spring from the pilot's inner sill support span.
Note: It's easier to do this with the GP40 (as shown in the example below) than on the SD45 because there is more room between the truck and pilot's end sill, but it can be accomplished by rotating the SD45 truck so as to pass the spring between the truck motor block and wheel's backside. Small long nose pliers will be a helpful tool to extract the spring as will be shown for the SD45 later.

Then remove the screw that holds the knuckle within the Aristo swinging arm assembly.

Then remove the coupler arm retaining screw and washer from the mounting post.

Then using a flat blade screwdriver, pry from side to side and front to back between the coupler arm halves to spread them apart as best as you can.

Keep in mind that the Aristo coupler knuckle is to be removed before the arm pieces are removed. Prying the arm apart a little facilitates this.

As seen in the below picture, note that the Aristo knuckle jaw is in its closed position - thereby allowing more room between its uncoupling lever and arm for easier removal.

Now remove the knuckle from the swinging arm by up and down motion until it comes out from between its arm.

Now remove the upper half of the swinging arm. This will require some manual force against the support span using your thumb & fingers, but it will flex just enough to facilitate the removal of the upper coupler arm half.

Followed by removing the lower arm half:

Withdraw the MU plug from the pilot opening if your GP40 was built this way as my example here was.

This harness will get in the way of the new Kadee coupler box when trying to install it and was a factory assembly oversight as they should have had the wires emanate from the upper holes as shown above and below where I poked in the brown wires.

If using this MU feature for battery power, etc., the plug will have to be removed and reattached once its wires are routed through the upper pilot holes as they should have been done by the factory like that previously done with the SD45 - shown below

The SD45 rear pilot from an SP example is shown in the above and below pictures. Note how the factory routed the wires for the MU plug. This should have been done for the GP40.

Rerouting the wires looks to be an awkward task as the factory obviously would do this before they attached the pilot's end sill to the chassis. However, now is the time to do it before installing the Kadee coupler box. Alternatively, you could leave the MU plug hanging out of the pilot opening, but spread its wires apart to allow for the box to be installed.

Shown below is the coupler mounting post once everything is out of the way.

Kadee 789 coupler box modifications

Below is a drawing of the Kadee 789 coupler box having modifications that are required for installation on the Aristo GP40 and SD45

Before arriving at the changes shown in red of the above drawing, I went through a succession of evolution stages that will be shown below. So keep in mind that some of the coupler boxes in the pictures may appear a little different than that shown in the drawing.


Shown below is the Kadee 789 coupler box.

The large nub recess area is where the loco's mounting post will pass through - albeit of smaller diameter.
It is important that the hole drilled in this area be tangent within the recess that is adjacent to the smaller hole. To assure this, I used a # 4 nylon washer placed within the recess touching wall next to the small hole as a pilot hole guide. The washer's outside diameter is to be close to that of the desired finish hole - namely 15/64 inch. The pilot hole, of course, is smaller as can be seen in the below picture as the smaller 7/64 inch drill bit passes through the washer's inside hole. You may need to jamb something in the crevasse near the box end of the recess to keep the washer in place whilst drilling - though I used my finger nail to hold it in place.

Once the pilot hole is located in the proper place, use a 15/64 inch diameter drill to finish with the proper hole size as shown below.

As shown below is the end view of the coupler box shank. Note the ridges.

Remove the ridges. I used a razor saw and file to clean up any protrusions.

Now flipping over to the other side of the shank, I took a slice (within 0.030 inch thick) off down the shank using the razor saw and cleaned up with a file.

With the ridges and slice removed, the shank is to be trimmed and cleaned up to 0.0140 inch thick.

Notch out the small hole area that is located near the back of the coupler box as shown below. The notch should be about 0.100 inch wide. I used a pin vise to help remove (drill out) some of the material. IMPORTANT: If the notch is too small, the coupler box will not fit over the mounting post's front rib - causing misalignment and the box to wobble.

Then finished up by trimming with the razor knife

Slice off (within 0.030 inch thick) the top of the coupler housing portion of the box to within 0.100 inch of its end.

Note: Since the thickness of the box in this area was measured at 0.087 inch, removing 0.030 inch leaves almost 0.06 inch left. This is sufficient given there remains the original 0.087 thickness at the last 0.100 inch at end of the box as the slice removed came to within this amount. Furthermore, to my recollection, some folks have chosen to lower the GP40 about 1/16 inch believed to be for prototypical reasons. If this is done, the additional material required to be removed from the coupler box will be too much. In such a circumstance, I believe it best to trim the loco's pilot lip, etc. to accommodate installation of the 789 box.

See below pictures of tools used to remove the material ...

...including the pieces as shown

The following is optional for reusing the Aristo centering spring:

Since the Kadee 789 coupler box will have minimal side to side travel within the pilot opening, there is little to gain by doing this if compared to the Aristo coupler, but the spring is useful to act as an upward vertical preload to the installed Kadee assembly. Since the spring is made of a small size wire, a pin vise with a very small diameter drill can be used to drill through the end face of the coupler box lid.

The centering spring installation will be discussed later.

Assemble the centerset coupler with its Kadee supplied coil springs into the box along with the lid.
The coupler is to be installed upside down to that typically done with Kadee couplers - particularly with respect to the instruction sheet that comes with the Kadee 789.
The bevel cut on the shank end as shown can be made before or after assembly. (Though tempting, I chose not to cut off the entire area where the small hole is located in order preserve strength since the slice cut was made to thin the shank.)

With the coupler box lid firmly screwed in place, the sharp tip of the screw will protrude.
It must be filed flush with the surrounding surface - otherwise upon installing the coupler box assembly, it will not seat properly against the pilot's front lip surface resulting in a wobble with misalignment.

With coupler box assembled with its lid screwed down, it should measure 0.350 inch across the box end as shown below

Installing the modified Kadee 789 coupler box assembly

Insert the modified Kadee 789 coupler assembly through the front of the pilot by first tilting it at an angle toward the top of the loco (the towel area in the below photo) so that the box part will clear the mouth opening surrounds of the pilot. During the insertion, the inner sill span will tend to interfere with the shank of the coupler assembly. However, by using your hand and fingers to flex it some, the coupler shank will eventually pop over the mounting post so that it passes through the drilled hole. The picture below shows the coupler assembly before I cut the bevel on the tail of its shank, and I was still able to install it that way. The bevel cut helps for this installation.

The picture below shows the coupler installed with the mounting post now passing through the hole of the shank.

Since the mounting post protrudes past the shank, a washer or spacer is required to support the coupler so it will assume the proper horizontal attitude and proper coupler to railhead distance once the loco is turned over and placed on a track.

I used a tight fitting nylon washer with a cut so it would not interfere with the box once placed over the post. It's important that this washer be thick enough that so that the installed, modified coupler box will fit without too much vertical movement - otherwise it could become dislodged during operation as happened on an SD45 that will be discussed later.

Alternatively, as shown below, a washer can be fabricated from an unused Kadee coupler box lid using the existing small hole to pilot the larger 15/64 inch diameter drill. Like cutting the washer previously described, the area chosen to be drilled out serves to not interfere with box once the fabricated washer is placed over the post. I tried this method on an SD45 that will be discussed later.

The round washer as seen below is installed using long nose pliers to hold it whilst being placed over the mounting post. Once positioned, you can use a small screwdriver blade to push it over the post before releasing the pliers.

The tight fitting washer as seen installed over the mounting post is shown below.

Similarly, install the original Aristo mounting screw with its metal washer over the whole assembly. Using small long nose pliers to hold the washer with screw whilst passing them under the sill span makes it easier to do.

Tighten down the screw for a snug fit.

Note: Depending on the nylon washer thickness that was placed over the Kadee shank on the mounting post but under the retaining screw and metal washer, the Kadee coupler assembly may not move or it may be allowed to move slightly up & down and from side to side as limited by the pilot opening. Either way is OK. However, if the coupler is allowed to move, the original Aristo knuckle coupler center spring can be put to good use.

Coupler centering spring - optional installation

As previously discussed, re use of the original Aristo knuckle coupler centering spring is not considered mandatory.

However, if used, it must be slightly modified to have a desired effect by putting some additional bend in it similar to that shown below.

Install the centering spring on & through the inner sill location as it was originally done for the Aristo knuckle coupler whilst passing its point end through the pin vise drilled Kadee cover plate lip as shown in the  following pictures:

The spring's tip will project such that it will restrict the Kadee coupler knuckle from freely swinging.
I bent the excess tip projection using a needle nose pliers as shown below. Alternatively, cutting off some excess can be done, but if for any reason one wanted to reinstall the Aristo knuckle coupler assembly, the spring will become too short.

Note how the spring tip is bent such that it will not restrict the Kadee coupler from swinging whilst providing an upward force on the outer end of the coupler box - beneficial for maintaining a horizontal level if the box is not tightly secured to the mounting post and allowed to freely move.

Note how the installed GP40 Kadee front coupler with its modifications and centering spring closely aligns with the Kadee 880 gauge.

Shown below is how the modified Kadee 789 coupler assembly looks behind the pilot's inner sill area once installed on the mounting post. The purpose of the bevel cut and slicing off the coupler shank ridges is solely done to facilitate its installation. The centering spring being bent as seen fastened to the inner sill span that projects out to the coupler box end face serves to provide an upward force here.

Shown below is the rear of the GP40 with its modified Kadee 789 coupler assembly installed.

Shown below is how the GP40 with modified coupler box (without having an upward force centering spring installed) as mounted with some freedom movement reacts with the Kadee 880 gauge with tension placed on it. It appears to work very well, too. Use of the spring merely adds an additional degree of assurance.

Securing unused MU cables

I found out the hard way that securing the GP40 MU cable must be done as just tucking it into the pilot recess did not work.
When I ran the loco on the layout the first thing that happened was the MU cable fell down and got caught into a turnout and acted like an aircraft arrestor hook when a plane lands on an aircraft carrier!
I used a tie wrap to strap the MU plug near the base of the pilot's inner sill as shown below.

The following are pictures of a Southern Pacific SD45 that show similar results to the GP40:

The alternative fabricated washer from a spare Kadee coupler box lid can be used on either the GP40 or SD45. The sharp outside corners could be nipped off - though not shown here.

Shown below is the fabricated washer as placed on top of the modified Kadee 789 coupler shank and over the chassis mounting post. Small needle nose pliers were used to put it there through the confined area that is more so with the SD45. 

The fabricated washer can be seated over mounting post with a small screwdriver

As shown below, the coupler box centering spring installation is facilitated by rotating the SD45 3 axle truck so that the spring is first placed between the motor block and wheel - then coxed with the pliers into the small hole on the inner sill supporting span. Before installing the spring, be sure to put some additional bend in it as previously described and shown for the GP40.

After the spring is coax through the hole at the front of the coupler box lid's lip, it can be tightened down - albeit with some difficulty as shown below.

The excess tip projection can be bent over with the pliers so it won't interfere with the knuckle from moving side to side as shown below.

Note how the spring tip is bent such that it will not restrict the Kadee coupler from swinging whilst providing an upward force on the outer end of the coupler box - beneficial for maintaining a horizontal level if the box is not tightly secured to the mounting post.
(Also, as shown below, the MU cable on the SD45 was properly factory installed through the upper holes in the pilot whereas the GP40 example was not.)

Note how the installed SD45 Kadee rear coupler with its modifications and centering spring closely aligns with the Kadee 880 gauge.

Shown below is the rear of the SD45 with its modified Kadee 789 coupler assembly installed.


Operational results

Tweaking the SD45:
It's important to observe what actually happens to the coupler action when the locos are loaded with a train as this will reveal any "tweaking" that may be needed.
To wit, a problem became very noticeable with the SD45's rear coupler when entering a wide radius turnout as can be seen in the below picture. (Note how the SD45's rear coupler box became dislodged as seen on the left.)

Investigating this showed a very subtle but slight warpage of the pilot. In this case, consequential tolerance differences resulted between pilot opening (it pointing somewhat downward) where the coupler box rests and the chassis mounting post. This required placing a thin washer between the modified Kadee shank and post. If I had known ahead of time, I would not have had to trim that 0.030 inch slice off the surface on this particular coupler - though not doing so would have made it more difficult to install.
Shown below is a 0.030 inch thick washer I used where the end must be trimmed off so it will not interfere with the back end of coupler box.
For this particularly problem fix, this washer is first placed over the mounting post, then the coupler box is installed.

The installed washer can be seen through the foot step opening in the below picture.
Doing this resulted in a securely mounted, horizontally aligned, coupler box assembly once it's fastening screw was firmly tightened down.
In this case there is no movement of the coupler box so the wire centering spring serves no purpose.
Since there is little clearance between the Kadee couple box and pilot opening, allowing coupler box movement is of little value unless one were to widen the loco's pilot opening at the sides. Doing so may allow operation on smaller than 8 foot diameter track but coupled rolling stock (or locos) would soon bind against the face of the loco unless the coupler shank were modified so the box sticks out farther forward.

Shown below is good coupler horizontal alignment using the Kadee 880 gauge.

Shown below is the SD45 on the left with its coupler box still held securely in place under load conditions on a curve track. The GP40 is on the right.

The other side is shown below with the SD45 on the right and GP40 on the left on an 8 foot diameter curve.


The GP40s:

The following pictures will verify more than sufficient room between the pilot ends of the locos when coupled together.
Below is a side view of the two GP40s on a straight track.

Shown below is a picture of the two GP40s coupled back to back on an 8 foot diameter curve track.

The close up view below shows the two locos with the couplers in tension.

The close up view below shows the two GP40 locos pushed together with the couplers in compression - an operational unlikely condition.

Below is an inside view of the two GP40 locos on the 8 foot diameter curve track with the couplers in tension.

Shown below is a GP40 with modified Kadee 789 coupled to an American Main Line (AML) brand SP double door box car retrofitted with a Kadee 830 body mount coupler assembly.

The loco and car shown here are on an 8 foot diameter track having plenty of clearance.

The GP40s and SD45:

Shown below is a GP40 on the left coupled back to back with the SD45 on the right on an 8 foot diameter curve. The locos couplers are shown in tension. As observed here, Kadee coupler action is such they actually pull together more so when in tension - unlike the Aristo couplers that are apt to pull away.

The close up view below shows the GP40 and SD45 locos pushed together on the 8 foot diameter curve with the couplers in compression - perhaps an operational unlikely condition. Since I have yet to mount Kadees on a second SD45 to try this on, it may be two SD45s when pushed together can touch at the pilot end steps.

Shown below are the two locos with their couplers in a relaxed state - but this time on a 10 foot diameter curve.

The inside view is shown below. Though the couplers of each loco lined up with the Kadee 880 gauge on my reference track, it is normal for some amount of coupler up and down mismatch due to track height irregularities as shown here. This varies as the train traverses the layout.

Also, note in the above picture, that the GP40 with modified coupler box assembly (shown on the left) did not include a centering spring whereas the SD45 on the right has the spring as can be seen through its pilot step opening. As previously described, the spring is not really necessary unless one were to open up the pilot end coupler pocket sides to allow for some coupler box swing if attempting to operate the locos on tighter than 8 foot diameter curves.

The locos coupled to a car in a test train.

A test train was made up of approximately 40 cars pulled by two GP40s fitted with the modified Kadee 789 coupler boxes. The train ran well without any coupler pull apart problems.

Shown below is an overhead view of the trailing GP40 coupled to an Aristo SP double door box car retrofitted with a Kadee 830 body mount coupler. This section of the train is on a 10 foot diameter loop back curve track.

Shown below is the trailing GP40 coupled to an Aristo 100 ton hopper car retrofitted with a Kadee 789 truck mount coupler. Again, the track curves are 10 foot diameter.

Though coupling locos with a body mount coupler to a car having a truck mount coupler can be problematic on curves, there was no problem pulling the test train with this arrangement here.
Inside view on 10 foot diameter curve shown below

Outside view on 10 foot diameter curve shown below

Shown below is perhaps the worst case situation with the SD45 coupled to a car.
The car is the same Aristo double door box car retrofitted with a Kadee 830 body mount coupler.
As can be seen there is plenty of room between the SD45 and the car on an 8 foot diameter track curve.

Aristo GP40s coupled to an AML brand car having AML coupler

Normally I retrofit Kadee 830 coupler assemblies to the AML cars, but as an experiment, I decided to try coupling the GP40 with its modified Kadee 789 to an AML stock car having its AML factory supplied body mount coupler installed. The AML body mount coupler assembly resembles the Kadee 830, but the AML box is slightly thinner, and the coupler knuckle end is bigger.

The knuckle also has an upward vertical offset from its shank centerline - shown below upside down.

As such I added spacers totaling about 0.080 inches between the AML box and the car's mounting pad so it would line up with the Kadee 830 gauge. Shown below are the mounting surface (lids) of the Kadee 830 and AML coupler box assemblies.

The Kadee and AML couplers will latch together - best done with the AML coupler jaw in a closed position.

Shown below is the test train with two AML (WP & ATSF) stock cars being prototypically correct as added to the front of the train.

 Note that the AML mounting pad holes are such that the box sticks out in a non prototypical fashion that is also unnecessarily far for operation on 8 or 10 foot curves.

Shown above and below is this section of the train on a 10 foot diameter loop back curve track.

Surprisingly, this train ran well without any coupler pull apart problems - even though the AML coupler looks to be marginal at its jaw area when mated to the Kadee coupler.
However, given the time and effort required to mount the AML couplers to line up and mate with the Kadees, I believe it best to mount the Kadee 830s from the get go and have all rolling stock work together with consistent harmony - significant when taking advantage of the Kadee magnetic uncoupling feature.

Another test train - two Aristo GP40s and one SD45 with Kadees:

Shown below are a GP40, SD45, and GP40 all retrofitted with the modified Kadee 789 boxes coupled together pulling a 42 car - mostly reefer block train. Their couplers did not pull apart as had been the case with the Aristo GP40 knuckle couplers. The two GP40s can pull the train without the SD45. The SD45 was added for the test and for affect. The 3 locos operated extremely well together being relatively speed matched. They sound very harmonious.

The below picture is precursor to a video made of the train in operation.

Here's the video:



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