USA Trains GP38 - Kadee Centerset Install, Abandoning Traction Tires &  Various Mods

USA Trains GP 38 - Kadee Centerset Coupler Install, Abandoning Traction Tires & Various Mod's
Ted Doskaris
March 23, 2012
Rev GE-A

March 24, 2012
Rev GE-B  Exhaust stack info. added

March 27, 2012
Rev GE-C  Added Loco height & coupler box projection info.

Sept. 17, 2013

Rev GE-D  Added subsection for Altering the GP 38 Pedestal to accept coupler box; Added Appendix A for motor current in-rush resistors

January 7, 2017
Rev GE-E  Added Appendix B, Lamp illumination characteristics


Engineer in cab at detailed control stand of USAT GP38 is very impressive


Topics covered include the following:

The USAT GP38 diesel in Rock Island Blue Livery - factory boxed
Centerset Couplers
Traction Tire and Wheel Size Issues
Solutions - Raising up the GP38
GP38 Pilot Modifications and Kadee Centerset Couplers
GP38 Disassembly
Replacing an Axle - Wheel assembly
The Coupler Mounting Pedestal
Notching The Pilots
Plugging The Excessive Pilot Opening
Installing Detail Parts
Coupler Alignment
GP38 Measured Weight
Layout Operation

Appendix A Motor current in-rush resistors

Appendix B GP 38 Lamp Characteristics

Videos -


The USAT GP38 diesel in Rock Island Blue Livery - factory boxed

The USAT GP 38 example to be described is in the Rock Island blue livery - the attractive but last paint scheme the Railroad used prior to its final bankruptcy and liquidation in April 1980.

The GP 38 comes standard with hook & loop couplers mounted - as seen in the pictures below having taken the loco out of its foam packing.
Note the large slot opening in the pilot required for this type coupler to swing.

Centerset Couplers

I obtained my first USAT 4 axle diesel loco - the GP38 in Rock Island blue livery.
I chose to use Kadee centerset type couplers because they are best suited for pulling heavy trains since they will avoid levering downward under load like the offset version Kadee delegated for this loco. Centersets, also, look prototypical.

In order to install Kadee centerset type couplers, some simple modifications are required.
Though Kadee plastic boxes could be used, albeit with some dimensional differences, I chose to use metal coupler boxes fitted with the couplers, springs and lid from the Kadee 907 kit.
These are the same CNC machined coupler boxes I had specially made by Datum Precision, Grass Valley, CA, for use on several Aristo-Craft locos - described in another Vignette.

Traction Tire and Wheel Size Issue

Prior to getting the GP38, I did get two Southern Pacific USAT SD70s since they were factory made not having wheels with traction tires. So I purposely got those SD70s as my first USAT locos.

Generally, I stayed away from getting USAT locos because of the traction tires they would come with. Though, as the name suggests, the tires do afford a light weight loco to have good pulling ability, but I don't care for them because they trade-off risking electric motor damage under high load conditions that slipping wheels would tend to avoid.
Moreover, having some amount of wheel slipping minimizes axle binding forces imparted to the plastic gear collars when operating the loco on track curves. Also, the ability of wheels to slip lends itself for MUing to another loco having slightly different speed characteristics.
Another consideration is the diminished electrical pickup from the rails with wheels having traction tires for track power users like myself. To wit, necessitating augmented electrical pickup points from the rails is why I suspect USAT locos typically include slider shoes.

As to wheel size, the GP 38 wheel is non prototypically small in tread diameter.

For a 1/29th scale loco, this wheel scales out to about 32 inches in diameter, whereas, the prototype GP 38, GP 40 and other road diesels use 40 inchers or greater. Consequently, the USAT GP 38 (along with other USAT locos using the same truck) have relatively tiny wheels that would be most appropriate for a small switch engine like the GE 44 tonner !

The best solution is to replace the wheels with ones of the proper diameter - maybe like those supplied with the USAT SD 40 that scale out to about 39 inches in tread diameter; however, this approach presents its own complexities when trying to install them - left for another, perhaps future, endeavor.

Thus, for the time being, I chose to raise the loco so that it's more prototypical for overall height from the railhead.

Solutions - Raising up the GP38
Raising the loco is, also, behooving when installing Kadee centerset couplers.

An evaluation was first made for how much the GP38 should be raised up by referring to the prototype GP38 and GP38-2 (and GP40) dimensions.  These were determined using the following book:

BNSF Railway Company 2006 Locomotive Review and Locomotive Diagrams<br />
By Robert C. Del Grosso,
Published by Great Northern Pacific Publications,
Bonners Ferry, Idaho 83805
(ISBN number not apparent)

Drawings and listed dimension for the GP38 are on page 139, and the GP38-2 is on page 140.  A GP40 is on page 144.

Loco Height Measurements

The book drawings indicate these 3 locos all have the same "extreme height" specification from the railhead of 15 feet, 5 inches.

When I measured the model locos, the USAT GP38 was notably low whilst the Aristo GP40 was not too far off at about 6.25 inches (which scales up to 15 feet, 1 to 2 inches) so the GP40 appears to be an appropriate comparative reference.


Though some folks may argue the "10 foot rule" would not make the loco look much different, but when trying to install coupler boxes with centerset couplers the height is important for how much the loco's pilots are to be notched out for the coupler boxes - as they must be to some more or less extent - with less being preferred if the loco is raised up. Moreover, if the future beholds using larger wheels instead of the spacers, the amount of notching would be very close for that application, too.

The following drawing and pictures serve to encapsulate what is involved to raise up the GP 38 loco using a spacer placed between each of the truck's A frame and chassis:

The completed loco having been raised with spacers, lowered fuel tank, pilot plugs and other detail parts installed is shown.

GP38 Pilot Modifications and Kadee Centerset Couplers

The following pictures serve to encapsulate what is involved to modify the GP 38 pilot area, particularly with respect to installing Kadee centerset couplers:

As alternative to making a plug to cover the large pilot opening, USAT does offer an optional snow plow that should cover much of the opening . This snow plow (part no. R2021) is used on several other USAT locos, too, and has an MSRP of about $18.00. So if you wanted to cover both ends of the loco, it would cost twice that amount.


GP38 Disassembly

In order to install truck spacers, the loco's shell must be removed from the chassis so that the trucks can be unfastened and removed. The pictures below illustrates where and how to remove the screws.

The trucks can be pivoted such that screws in this area can be reached with the screwdriver placed between the motor block and its side frame. Also, there are more screws hidden by the fuel tank - so it has to be removed.

Shown below is a removed truck.

Shown below is how side frames are attached.

The side frame stirrup covers up the side frame mounting screws. If the stirrups are already installed they can be removed by prying them away from the side frames.

Since the side frames hold the whole truck assembly together, if so desired, the motor block can be withdrawn after the side frames are removed.

Replacing an Axle - Wheel assembly

Shown below is how a motor block axle is removed and replaced (R&R).

With the cover off of the motor block, the slider shoes can be lifted out if you don't want to use them since they have no fasteners.

When reinstalling the lid, both axle assemblies must be held down against the spring tension of the long shinny wires. All axle brass bushings must be kept square and parallel with the plastic motor block housing surface while being seated within the cups. To do this, you can hold one of the axles in place with a rubber band so that both hands are freed up - with one hand then used to hold down the replacement axle whilst the other hand is used to manipulate the lid into place on the motor block - or hold both axles in place with a rubber band for each of them, freeing up both hands to install the lid.

The Coupler Mounting Pedestal

In order to mount the Coupler box (Datum Precision metal or Kadee plastic), the pedestal used for the standard USAT mounted hook & loop coupler must be altered or replaced. The hook & loop coupler is shown below.


To accept Kadee centerset couplers, the mounting pedestal can be modified or replaced with a fabricated one.


Altering the GP 38 Pedestal to Accept Kadee Centersets

The following diagram and pictures show modifications to be done to accept the Datum Precision metal coupler box; however, if using Kadee plastic boxes, the height of the pedestal will need to be trimmed down a bit more. Modifications for the loco's front and rear pedestals are identical since they are the same.


Note that the plug may project somewhat below the base surface of the pedestal because of the way the mounting ribbing of the loco's chassis is designed. This can be used to advantage if you want to add a third mounting screw, albeit from the interior side of the loco; but the shell would have to be removed to do so.


The picture below shows the unmodified pedestal on the left and the modified pedestal on the right.

After trimming the height at the top of the pedestal there is very little material left to mount a coupler box. Note the plug is to be fully seated into the hollow area of the factory pedestal and bonded using plastic glue.

The modified pedestal can now be mounted to the loco's chassis. Note that notching the pilot must be done to accept a coupler box having centerset couplers. The method to do this is discussed later.

A shim may be needed to align the coupler box assembly for proper coupler height to track rail head.

The picture below shows the mounting is now completed.

The coupler now aligns with the Kadee 980 track height gauge as shown below.


Making a new Pedestal

A new pedestal assembly can be made as an altenative of having to fill the factory hollow pedestal with material when it is cut short to a required less tall height.

With the tentative height pedestal, I used a couple of stacked washers (shown below) to get the proper railhead to coupler height and leveling. Future version pedestals heights will be dimensionally better "zeroed-in".

Notching The Pilots

The front and rear pilots must be notched to accept the Datum Precision metal or Kadee plastic coupler box when using centerset type couplers. I used the Datum Precision metal coupler boxes.

Coupler Box Projection

The Aristo GP40 was emulated as to its molded-in pilot fairing and coupler box fitment.  I used the Datum Precision metal coupler box; however, a Kadee box should work on the GP38, too - though pilot notching depth and pedestal height will be somewhat different with the Kadee plastic box.

The coupler box projection is set at a distance of 0.400 inch as measured from the flat surface of the pilot face.

The draft gear fairing block was made to fit on the top of the projected coupler box.  Since the Datum Precision metal box is made with a lip, the block was cut a bit shorter at its base.

Plugging The Excessive Pilot Opening

With the pilots notched, a plug is fabricated for covering the excessively large opening in the pilot.

Also, for realism, the pilot should have a transitional fairing for the draft gear placed on top of the projected coupler box. The following figure serves to illustrates how these things can be done:


Installing Detail Parts

USAT provides detail parts for the customer to install on the loco that the factory does not do.

Note that some plastic and metal parts described in the pictures to follow will not fit too well into their intended chassis holes because of factory paint buildup, so the the paint is to be removed as required (A/R).

Pilot Ends

The following pictures serve as a method guide for installing parts on the front and rear pilots:

Pilot End Railings

Side Railings

Exhaust Stacks

Coupler Alignment

With the GP38 completed having been modifed (raised up with Datum Precision metal coupler boxes employing Kadee 907 centerset couplers), the picture below shows coupler alignment with the Kadee 980 gauge.

GP38 Measured Weight

With all parts installed, the completed GP38 measured 7.9 pounds.

Layout Operation

Shown below is the USAT GP38 as the trailing loco with two SD70 lead units pulling a 62 car train on the under house layout. The GP38 is equipped with Kadee 907 centerset couplers installed in custom designed metal coupler boxes that I had made by Datum Precision, located in Grass Valley, CA. These coupler boxes were intended for use on many Aristo locos, so this is the first application on a USAT loco.

Shown below is an overhead view of the GP38 coupled to the first car of the train - an AML brand BN BETHGON equipped with Kadee centerset couplers. This part of the train is on the 10 foot diameter 270 degree loopback.

Shown below is an inside view on the track curve of the coupler action undergoing the train load stress

Shown below is an outside view on the track curve of the coupler action undergoing the train load stress.

Shown below, the smooth surface clear Lexan truck spacer can be seen retained in its chassis recess. The fabricated coupler pedestal can, also, be seen.

Shown below is an overhead view of the SD70 coupled to the GP38. The SD70 is equipped with a Kadee 787 medium offset coupler adapted to a swinging box with spring return - described in another article.

Shown below is an outside view on the track curve of the SD70 and GP38 coupled together with the stress of the train load.

The stress load of the long train affects the coupler action differently on other parts of the layout. Shown below is the GP38 coupled to the same BN BETHGON car on a straight track section - one of the few non curve areas of my layout. In this example the couplers are better aligned.


Videos -

The video below (14:5 minutes) includes a description of GP38 modifications followed at 9:15 into the video by operation with 2 SD70s pulling a long train on the under house layout.


The video below (13:5 minutes) was done prior to the modifications, but includes a description of wheels sizes, loco height to the rail head and comparison of the GP38 to the Aristo GP40 for operating speed and current draw.




Appendix A - Adding Motor Current in-rush Resistors


When an electric motor first starts from a standstill state, a very high initial current results when power is applied. As the motor spins, sufficient back electro motive force (Back EMF) results to limit the current under normal operation. When using track power, this high current can cause undesired effects, such as wheel pitting and dirty track work, particularly if using Aristo's pulse width control (PWC) for track power since voltage is applied in an On & Off fashion at a repetitive rate.

George Schreyer investigated this issue in great detail. See his Web site section on Wheel Pitting.

To minimize the high in-rush current a one ohm resistor can be placed in series with each motor. The trade-off of adding the resistors is to slow the loco's speed down slightly. Since the GP 38 seems to run a bit fast for my tastes, I found this not to be objectionable - even desirable - since the GP 38 can be a better speed matched when MUing to an SD 40 or even an Aristo GP 40.

I currently have two GP 38s, albeit with the same road numbers.

I configured one loco with a Double Pole Double Throw (DPDT) switch (placed in the fuel tank) so the resistors can be engaged or not used (normal). The schematic diagram for this is shown below.

With the loco's shell and fuel tank removed, I merely plugged the circuit into the existing path of the motors using connectors (with pigtails) purchased from RLD Hobbies.

The resistors are fastened on a thick aluminum block placed over the rear weight within the loco. The screw holding the rear weight was removed and replaced with a longer flat head screw so both aluminum block with resistors and the weight are together held in place.

The following figure depicts the detail of this installation.

I configured the other loco with just the resistors. This is much simpler to do since I was satisfied with the operational results and feel no need to revert to the original configuration having no resistors. The schematic diagram for this is shown below.


With the loco's shell removed, I merely plugged the circuit into the existing path of the motors using connectors (with pigtails) purchased from RLD Hobbies.


Shown below is a 14 minute video of the two locos pulling a Rock Island blue  train on my layout having all modifications, including the motor in rush current resistiors.




Appendix B GP 38 Lamp Characteristics

The GP 38 version I have includes both incandescent and LED lights.

The front and rear headlights and number boards are incandescent.  The front and rear classification lights are bicolor (Red or Green) Light Emitting Diodes (LEDs).

When operating the loco using DC Linear power the lights won't illuminate until sufficient voltage is applied even though the loco moves slowly.

When the loco is operated in the forward direction the two vertically stacked front headlights illuminate, and the two classification lights in the low nose illuminate green.  The rear highlights are off, and the rear classification lights illuminate red.

When the loco is operated in the reverse direction the two horizontally grouped rear highlights illuminate, and the two rear classification lights illuminate green.  The front headlights are off, and the classification lights in the low nose illuminate red.
The number boards are illuminated irrespective of operational direction but are so dim they are not very noticeable unless seen in a darken environment.


END -Ted

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