Rebuilding an old KTM/US Hobbies Southern Pacific AC-12 (Part 2)

Figure 0 – The finished model fully detailed and DCC equipped.

Our previous article left us with a very performing chassis, providing this old KTM Cab-Forward with

an updated, powerful and silky smooth traction platform. The second installment of this “rebuild” will

cover the rebuilding of the shell, the addition of details, the installation of a DCC decoder and the

painting of the model.

However, before jumping in the details of the rebuilding, let’s provide a list of parts that were needed to

rebuild the chassis of the AC-12 and to re-motor it:

1) Northwest Short Line Pittman motor 40611-9 (Pittman 9434)

2) NWSL Flywheel 416-6 (Brass)

3) Precision Scale Co Universal Joints

4) Small Ball Bearing 4mm internal diameter

5) Re-use of one of the original KTM motor mounts.

6) Small brass strips to hold the new motor.

7) Three 3mm bolts and nuts.

8) Small wood block to hold the motor (shaped).

As shown below, the chassis flying by the station of Bealville on the beautiful layout of Jerry, deserves a

nice and updated shell to take full advantage of its incredible running qualities. The idea was also to

install a DCC decoder so as to control, Headlight, Tender Light, classification lights and of course, the

motor.

Figure 1 – The complete re-motored chassis passing by Bealville, CA on Jerry’s layout.

Let’s start with the installation of the DCC decoder first. My system is a Lenz DCC and as such I like to

use the Lenz Gold Maxi 10440 decoders. Other decoders could be used, but I find the Gold Maxi very

practical because of its mounting holes and mini-connectors. The motor does not use much more than

1.5 Amps under full load and will peak at about 5 amps when stalling the engine. The Gold Maxi with

its 4 Amps capability is a nice decoder and provides all the flexibility that you can dream about. Up to 8

light outputs can be wired, which is much more than we need. As we will see in the final section of this

article, this Lenz decoder turned out to be undersized for the true capability of the beast, and the NCE

408 would have been a better choice.

The cavernous boiler of the AC-12 is the perfect place for mounting such DCC controller. The good

thing is that this little Printed Circuit Board will easily fit on top of the Pittman motor, as shown in the

following pictures (2 & 3).

On one side, a small square tube (K&S) is soldered to a small brass vertical plate taken in sandwich with

the mounting braces of the motor. On the other side, two small bras strips are used to hold and secure the

DCC decoder to the front of the motor. The resulting mount is very sturdy and allows close wiring of the

motor to the DCC with beefy electrical wires. The whole enchilada is completely removable and can be

taken apart in a few seconds, thanks to the electrical connectors, in the unlikely case of a decoder failure.

In fact the Lenz decoder is thermally and current overload protected and will shut down if pushed to its

limits.

If extra cooling is required, a small brass place could be mounted on the back of the power transistor,

above the decoder, and would still fit in the humongous boiler. That is the key factor about these can

motors: they take so less space than the old open armature motors, that a lot of things can be added to fill

up the space previously used by the original KTM motor.

Figure 2 – The Lenz Gold Maxi DCC decoder sitting on top of the motor.

Figure 3 – The Gold Maxi DCC decoder mount.

Before going further, verification that the motor and its piggybacking DCC decoder fit into the boiler

must be done:

Figure 4 – Everything fits into the firebox.

Not only does everything fit perfectly, but the engine is extremely silent, which is very encouraging. We

will see later how we can even reduce the noise of the locomotive by coating the inside of the boiler

with a thick paint.

After installing the decoder, let’s move to the tender and equip it with rear light and classification lights.

Here we use a 3mm white LED and two 2mm red LED (purchased from Digi-Key). The lights are

connected to a separate connectors located on the front of the tender with 6 contacts (again Digi-Key).

Two contacts are used for the Rear light, two for the classification lights and two are for future use, such

as a speaker. The following pictures show the wiring inside the tender (5 & 6). The White LED is

nominally working with 2.3 volts under 20 mA. The DCC decoder provides about 12 Volts of voltage.

We need a resistor of (12 - 2.3) / 0.02 = 485 or 470 Ohms (standard value). For the classification lights,

we use two red LED wired in series, each working under 2 volts/20 mA. Again using the Ohm law, we

must wire in series a resistor of (12 – 4) / 0.02 = 400 or 390 Ohms standard value. If the classification

lights are too bright, you can use a 420 Ohms or even 470 Ohms. In all cases, the total current drawn by

the lights on the tender will never exceed 40 mA, which is far away from the 1000mA that the Decoder

is capable of providing for the light outputs. The original KTM connector is kept for the power

connection to the engine.

Figure 5 – Connector with 6 contacts on the tender.

Figure 6 – Wiring of the rear light and classification lights.

For the locomotive, I endlessly debated about wiring the front classification lights or not. I could have

done it, but did not like the fact that the original holder would have had to be replaced for something a

little bit larger and the prospect of having these electrical wires and their insulation visible on the front

of the cabin did not appeal too much to me. Decision, decision…. I finally decided to install jewels

instead.

Also, the original front light was broken and I could not find a replacement (besides, replacing this bulb

means that you have to unsolder the ring that holds it, remove the old bulb, replace it and solder back the

ring. That could have been messy and the chances to burn that bulb again, were not slim. Therefore, a

while/yellowish 5mm LED was used instead, and installed in a small 5mm brass tube soldered from

behind. The lens was cut and carefully polished from a piece of clear acrylic to fit into the original ring.

Now that we have the lights figured out and the DCC decoder installed, we need to wire these together.

Wiring on a regular steam engine with a rear cabin and a fixed (read straight) chassis is relatively easy.

With an articulated and even worse a Cab forward engine, it can become a nightmare to route all these

electrical wires from the front of the engine to the rear of the tender. And because we have an articulated

locomotive in our hands, it is out of question to run electrical wires inside the boiler. Everything must

travel underneath, and be hidden or at least disguised in water, steam and air pipes. The following

pictures are showing what was done.

The picture 7 shows the wires coming down from the DCC decoder (right side of the picture) and being

neatly organized and secured underneath. The bottom wire is used for the power coming from the left

wheels of the tender, while the thinner top wires are used to power the rear and classification lights of

the tender.

Figure 7 – Power and Lights wires at the front.

The next picture (8) shows the articulation of the Cab Forward. Here, it is essential to make sure

that the rear chassis can pick-up electrical current and sent it to the front chassis and to the DCC

decoder. The wire must be long enough to allow the articulation to work both ways and to offer

some longitudinal play, required when re-assembling the chassis against the massive boiler.

Figure 8 – Electrical connections on the articulation.

Figure 9 shows the wires coming back up to the surface after their long journey underneath, and

being connected to the tender as oil and steam or air pipes. The shrink tubing will be heated,

providing a neat appearance and an almost seamless continuity of the pipes. These wires lead to

the 6 pins connector we saw on the tender, before.

Figure 9 – Power and Lights wires in the rear.

Figure 10 – Connections to the tender disguised as steam/fuel/air conduits.

At this stage, we have a nicely and smooth running chassis equipped with a DCC decoder. It is

now time to move to the most difficult part of the restoration of this old KTM engine: the shell

and the painting.

When the engine was acquired, it was in pitiful condition. The paint job was far from being of

good quality, lots of parts had been glued instead of being soldered and the decals were showing

like the nose in the middle of a clown’s face. I wanted to replace some broken parts (ladders) and

re-solder important pieces that were partially broken, such as the walkways on each side of the

boiler. The railing on the rear platform was damaged as well as a few parts around the cab. There

was no other solution than to strip entirely the model from its thick paint, repair carefully all

broken parts, add some details and replace all ladders and steps.

Figure 11 – Six weeks of brake fluid soaking will make you shiny…

The long shell/boiler was soaked for 6 weeks into pure brake fluid. It took me four quarts to

cover entirely the boiler in a long plastic box. The paint was probably gone in a shorter time, but

I was busy with other projects and did not mind forgetting tender and shell into the fluid. The

brake fluid was carefully wiped away, and the shell and tender were left for a few days of

soaking into isopropyl alcohol. This second bath did remove all traces of brake fluid and made

the brass very shiny and clean. At this point, repairs and detailing could be tackled seriously.

The ladders leading to the cab, were partially broken: rungs were missing or had been

unsoldered. I tried to repair them but quickly realized that my efforts would be doomed as

soldering one rung caused the other ones to fall apart. Fortunately, Precision Scale Co. has the

Cab Forward ladders in their steam catalog and the wonderful thing is that these ladders are cast

in one piece. I am still wondering how these Japanese wizards at KTM, could assemble these

ladders without loosing their sanity.

The handrails to the cab were soldered back in place: the idea was to not have any glue holding

brass parts. I used a micro-torch to re-solder the walkways along the boiler, as these parts require

a significant amount of heat and had to be very solid, as the engine is always held there when

taken out of and on the rails.

The original KTM/US hobbies engine is more an AC-11 or AC-10 than an AC-12. It is missing

major pipes on the left side mostly and I tried to gather photographic information to make my

model as close as possible to the AC-12 prototype.

An excellent source showing the pipes arrangement on an AC-12 are the few pictures of the 4294

from the Sacramento Railroad museum floating on the Internet, and some historic pictures. The

following picture shows clearly the lagged pipes on the left side of the cab forward.

Figure 12 – SP4294 Pictured in Sacramento Railroad Museum

I decided to add one additional steam lagged pipe and slightly reroute the existing ones as

pictured below. I also added another pipe running next to the handrail on the left side of the

boiler.

Figure 13 – The lagged steam pipe, signature of the AC-12.

Figure 14 – Additional pipe lagged with fine ribbon.

The pipes were lagged with some of the smallest ribbon I could find, the type used in very

skimpy lingerie. The ribbon was held in place with AC glue and carefully wrapped to simulate

abestos lagging. The most difficult was to lag the vertical pipes on the side of the boiler as these

could not be removed. I had to cut very short strips and glue them carefully on the brass. It is not

easy and I must have spent hours installing this small ribbon. The results, however, is worth the

investment in time as shown in the final pictures, and gives the model a very realistic appearance

that the regular brass lagging found on some models, cannot even approach.

Figure 15 – Adding Pipes connectors.

The tender went through the same restoration as the main shell, as the steps, oil pump and other

details on the top were broken. The following pictures (Figure 16 & 17) show the tender, after its

six weeks bath, ready for painting. The pink goo is used to protect the LEDs from the invasive

spray of the airbrush.

Figure 16 – The tender shell during its restoration.

Figure 17 – The tender shell during its restoration (rear view)

After stripping the old paint and cleaning up the shelves, it is now time to proceed with some

extra detailing. As we all know, the early KTM and US Hobbies Japanese imports were not

highly detailed by comparison to what KTM offered later on (under the name of West Side and

KTM USA), or what Precision and even Sunset/3rd

Rail offer nowadays. Therefore, after

spending a great deal of time re-motoring this platform, it was time to dedicate a few hours to

adding new details, details that would be visible and characteristic of the Cab Forward AC-12

engines. We have already talked about the most obvious and visible details, the extra piping and

its lagging on the left side of the engine.

When it comes to detailing and painting, the archive pictures are kind of useless. I do not know

about you, but when you try to decipher small details or shades in these black and white pictures,

it is very hard to get anything out of them. Most of the times, they are too contrasted to be really

useful, and besides a few rare pictures that you find in some good quality books, most are useless

if you are trying to route a specific pipe or find out how many lagging clamps are mounted on a

specific engine. Also, living on the East Coast, it was hard to impossible for me to travel to the

Sacramento Railroad museum to see the AC-12 for real. Fortunately, the AC-12 is an engine that

has been reproduced quite a few times, and Broadway Limited recently came up with a 1/32nd

outstanding model, which is in my humble opinion, the best you can find around. Unfortunately,

they never produced it as the economic downturn (Thank you Wall Street!) killed that project

and not enough reservation could be gathered to justify the series. However, a couple of

prototypes were manufactured and they represent a mine of details and perfection for the Cab

Forward enthusiasts. The following pictures show the complete model and some details of the

top of the boiler:

Figure 18 – The gorgeous 1/32

nd Broadway Limited AC-12.

Figure 19 – Details of the top of the boiler on the Broadway Limited model

Studying these pictures in great detail, I decided to replicate the piping around the steam electric

generator and to add the lagging clamps on the top of the boiler. Both of these details were

painfully lacking on the USH models.

The following pictures show the finished engine, in its primer dress with all the added details. At

this stage, I wished they had primer in black color, as it is so much easier to apply than any so

called “specialized” paint. One can dream…..

Figure 20 – Primed and ready for the final coats.

Figure 21 – The trademark of the AC-7 to AC-12 engines.

Figure 22 – The intricacies of the air pumps on the smoke box door.

Figure 23 – Lagging clamps and piping of the dynamo.

Figure 24 – The tender ready for its final spray.

At this point, I have been agonizing on how to best finish that engine. I had spent countless hours

rebuilding it, improving it, and detailing it. I did not want to ruin all this work with a lousy paint

job. I decided to go with Scale Coat paints, which are not the easiest to apply. The question was:

should I paint it as delivered from the factory or dirty and grimy from too much action?

One of the points that influenced my decision, was the resale value of a weathered engine. For

some silly reasons a nicely weathered engine will see its resale value considerably diminished on

the brass market. Another point was that I am not an operator as I don’t have a layout at home

and thus a weathered engine does not make too much sense for me, although I enjoy immensely

a nice and realistic weathering job on a steam engine. With the above in mind, I decided to paint

this model, fresh from the factory, as it would have been delivered to the Southern Pacific

Company or after a complete overhaul and re-painting of the engine. I also wanted to represent

one of the early AC-12 and decided to go with number 4278 (which happens to be the number on

the Microscale decals….how convenient). Now, I needed to find a picture of the prototype

#4278. After many hours spent searching the WEB, I finally found a decent picture on the Kohs

web site:

Figure 25 – The real 4278, showing its true colors and shades…

There is always a certain part of suspense and anxiety when painting an engine, and even more if

you have spent so much time preparing it and rebuilding it. For sure, I did not want to soak it

back into the brake fluid for a couple of weeks and start from scratch again. Thus, I made sure

that my Paasche airbrush was clean, that there was plenty of pressure in the tank and that the

Scale Coat paint was correctly thinned and mixed. I started with a first coat of engine black,

which is (contrary to Floquil paints) not flat at all, but rather on the glossy side. It was not a

problem as I would use it to apply the decals directly and always intended to apply a coat of flat

clear finish to protect the whole engine. I started painting the tender. I told myself: if I have to

ruin something, might as well be the tender, which is easier to recover than the main engine.

Everything went fine and my worries were not founded. I had painted other engines and cars

before, and the final result turned out to be very pleasant. After the black coat, I applied the

silver coating on the smoke box and the front of the cab. Here again, I used the Scale Coat paint,

which is horrible in silver. It takes very long to dry, and may react adversely with surrounding

paints. On the smoke box door, I wanted to paint the pipes and air pumps black….but no paint

would stick to that silver finish. I ended up using thinner on the tip of a brush, to carefully

remove the silver paint and expose the pumps and pipes that had been previously painted in

black. Some final and delicate touch-ups with a small hand brush, finished the model to my

satisfaction. I then applied the decals from Microscale on the front and sides of the cab and the

tender.

Finally, after a few days of letting the whole enchilada dry (I did not bake the paint, although I

could have done it, since all the parts were soldered with the exception of the lagging on the

pipes), I sprayed the final coat of Dullcoat to seal the decals and that “horrible” silver paint. To

my relief, the silver did not react with the Dullcoat and I called it a day….. Final hand touches

were made by painting some of the valves red.

Next phase was installing two seats for the crew and two small figurines to populate that

somehow plain and deserted cabin. I also added glasses to the side and front windows, by cutting

tiny pieces of 0.2 mm thick glass. The final rendering is ten times better than plastic, and looks

like the real thing, especially in the way it reflects light. A lens was also cut for the front light,

this time from acrylate and carefully polished to give it a convex shape.

The engine in itself is already very heavy. With the Pittman motor, I knew I could even increase

the pulling power by adding some extra weight. In the KTM engine, there is extra space between

the big weight located in the boiler and the boiler itself, and I decided to cover partially the

inside of the boiler with wheel weights used to balance the rims and tires on cars. These come

with sticking tape and are very easy to affix.

The original shell with boiler weight and paint amounts to 1980 grams plus 50 grams of pipes

and accessories. I was able to insert and add between the inside of the boiler and the original

KTM weight, a little bit more than 600 grams, bringing the total mass of the shell to 2630 grams,

or 5.8 pounds. To this weight, you have to add the weight of the chassis and motor. This

articulated engine is actually too heavy for my precision scales and I would estimate its weight to

about ………..

Much more weight could be added, but I decided to stop it there, because of the relatively heavy

mass of the complete engine and the risks to damage the parts when handling the locomotive.

Figure 26 – Adding weight inside the boiler.

The remainder of the inside of the boiler and firebox was painted with that black coating used for

pick-up truck bedding and sound insulation on automobiles. That chemical is nasty, and you

must use it outside if you intend to keep your lungs in working condition. The net result is that

the engine is so silent that when it leaves your sight and enters a tunnel, you have absolutely no

clue where it is. The extreme weight, the suspension of the drivers and the smoothness of the

motor make it a completely silent engine, almost stealthy.

This pretty much concludes the rebuilding of that beautiful engine. Let’s enjoy some pictures of

the finished model:

Figure 27 – Ready for that consist of reefers.

Figure 28 – The crew is on board.

Figure 29 – Left side

Figure 30 – Right side

Figure 31 – Details of the Smoke Box

Figure 32 – Clean tender

Figure 33 – Colossal power….

Figure 34 – The AC-12 with her older sister, the AM-2 (Sunset).