Category Archives: Unitah

Baldwin Unitah 2-6-6-T Superstructure Mods

This engine is being bashed from a Roundhouse Outside Frame 2-8-0 kit and a Roundhouse 3 in 1 kit.  The superstructure will primarily be made up of components from the 2-8-0 kit.  This superstructure project will have us doing the following:

  • Extending the 2-8-0 boiler.
  • Relocating domes.
  • Scratch building side tanks.
  • Bashing the cab to add a side door, shorten its length and add a bunker.
  • Extending the front pilot deck.
  • Detailing with brass parts.

Here is a Jon Davis drawing of a Unitah 2-6-6-2 articulated locomotive.

Unitah Mallet Davis Drawing

Extending the Boiler

Note that the front of the smoke box falls at about the midpoint of the front cylinders.  The following photo of the Roundhouse boiler shows the extent to which the boiler needs to be extended so that the smokebox falls in the right place.  By my calculations, the boiler needs to be extended by 0.8″.

2-6-6-2 Unitah Boiler Ext

A consideration in the boiler modifications needs to be that the underside if the boiler needs to be open to handle the drive train that will be powering both sets of drivers.  This image shows the underside of the boiler.  The drive train slot works fine for a 2-8-0.  Not so much for an articulated 2-6-6-2.  Note that the weight (silver object) is held in place by rivets.

2-6-6-2 Boiler Underside

The logical places to make the cuts to extend the boiler is shown in the following image.  One potential cut spot is right behind the smokebox (right cut line).  The problem with this location is that the splice will be fairly visible.  Another potential cut place is behind the right dome.  That cut/splice would push the dome closer to the front of the engine but the splice would be less visible as it would be hidden between the saddle tanks.

2-6-6-2 Boiler Cuts

In either case, the cut would be through the weight.  I’m considering relocating the weights to the saddle tanks, freeing up space under the boiler for the drive train.  At this point I prefer the right cut point as engine smoke boxes are often a slightly different diameter than the boiler because of the lagging applied to the boiler.  In addition, the splice may be far enough forward that the underside doesn’t need to be open to accommodate the drive train.

Plastruct makes tubing that is available in a variety of different diameters including 0.75 inches, the measured diameter of the Roundhouse boiler.  The Plastruct tube walls are 1/16 inch thick.  The next smallest tube (0.625″) slides tightly inside the 0.75″ tube allowing very solid support of the splice.  I may have to either taper a bit or shim a bit on the roundhouse sides of the splice depending on the thickness of the Roundhouse boiler casting.

So I cut the boiler at the right cut line and ended up with this.

2-6-6-2 Cut Boiller

All I have to do is make this work is:

  1. Remove the weights from both cut sides – doesn’t look easy.
  2. Cut a 0.8″ piece of the Plastruct tube and order the next smaller size so I can splice the three pieces together.
  3. Slot the bottom to clear the drive train.
  4. Cut off and relocate the domes.
  5. Whatever else is needed to make this work.

Or I could replace the entire boiler with Plastruct tube.  I would need to:

  1. Slot the bottom to clear the drive train.
  2. Place the domes where I need them – brass domes on order from Wiseman.
  3. Add a smoke box cover – on order from Wiseman.
  4. Fabricate a fire box – would need to be scratch built.
  5. Put bands on the new boiler – some styrene strips.
  6. Whatever else I need to do to make this work.

Somehow the latter sounds easier than the former so that is what we’ll do.

Saddle Tanks

Support of the saddle tanks is already cast into the Roundhouse boiler in the form of running boards. Or I will need to add supports to the new Plastruct boiler.  The saddle tanks will be scratch built from styrene.  Th hollow space inside the running boards will be filled with sufficient strip weights to more than offset the loss of weight inside the boiler.

Bashing the Cab

Not counting the bunker, the cab on the 2-6-6-2 is shorter than the Roundhouse cab.  With the bunker on the rear, there is no rear access to the cab so doors or door openings need to be added.  A consideration in bashing the cab and adding a bunker is that DCC and sound are anticipated. So the cab/bunker will need to accommodate a speaker and provide a space for the Soundtraxx Ecomomi DCC unit and sound circuit board.

Extending the Front Pilot Deck

Logging articulated engines had relatively long front pilot decks.  That was certainly true of the Unitah 2-6-6-2T.  There are a number of options for extending the depth of the pilot deck and putting a more prototypical front beam on the unit.

Detailing with Brass Parts

Wiseman just released an array of detail parts for HOn3.  I have a representative sampling coming to me.  Some of the parts are going to be used to give my engines a family feel.  Others are very specific to the Unitah Mallet including a deck mounted headlight, a potential front deck, Unitah front trucks, etc.  Check the Roundhouse Kits Detail Parts post for more details.

Baldwin Unitah 2-6-6-2T Frame Mods

I’ll be referencing the takeoffs that were shown on the page you used to navigate to this page.

Unitah Takeoffs

Also, the frame image on that page is the starting point for the discussion on this page.

2-6-6-2T Frame3

The takeoff table shows the spacing between the rearmost driver in the front block and the front driver in the rear block (Driver Front/Rear Wheelbase).  Using the drawing it was estimated at 1.016″.  In the bashed drive train it was initially estimated at 1.37″. We want to reduce that discrepancy.  In the above photo a portion of the rear frame protrudes from the cylinders.  In addition, in remeasuring, I found the cylinders were too far forward.  So I moved the rear cylinders back to a position where their distance from the front axle in their motor block would be the same for both cylinders.  I then cut off the protruding section of the rear frame and rounded the end so when the front driver block rotates to the right and left there will be no interference.

Unitah Frame Mod1

I also notched the frame where the cylinders will ride, but not as much as I originally anticipated.  I realized the the slide plate mounted above the front cylinders would raise the total height of that unit.  In this photo you can see where I filed the front cylinders to receive a brass plate that will allow the boiler to slide as the front driver block rotates.  The brass plate is not shown in this photo.

Unitah Frame Mod2

The slide plate is effectively a place for the boiler to slide and a shim bringing the saddles of the two sets of cylinders level with each other.  I have additional work to do on the drive train before making final adjustments to the relative height of the two sets of cylinders.  But here is how the drive train would look with the front and rear frame aligned as they will be once the frame is complete.  This is a much more compact unit than the image near the top of this page.

Unitah Frame Mod 3

This shot shows the front/rear driver block wheelbase.  The length of 1.111 inches is just a bit longer than the converted takeoff from the drawing which was 1.016″.

2-6-6-2 Front:Rear Whlb

While I had the digital calipers out I also wanted to measure the boiler extension needed for the front of the smokebox to fall at the midpoint of the front cylinders.  The calipers are telling me I need to add a 0.8″ extension to the boiler that comes with the Roundhouse 2-8-0 kit to meet that objective.

2-6-6-2 Unitah Boiler Ext

My next step was to clean out the wheel bearings.  No, your wheels are not going to drop into the bearings and turn nicely for a number of reasons.

  1. Your axles may be rusted.  After all, this is a 40 year old kit that was stored in somebody’s damp musty basement.  A little work with some very fine sandpaper will get them shiny again.
  2. The bearings have flash or were cast undersized.  A small rat tail file will solve this problem.  But go easy here.  You want an axle that turns freely but is not sloppy.  Make sure you have addressed the third problem before finalizing work on the second problem.
  3. You violated the Roundhouse 2-8-2 blind/flanged driver rule.  You see, on a 2-8-2 the flanged drivers go in the two outside bearings and the blind drivers in the two inside bearings.  If you are building a 2-8-2, simple.  Move the flanged wheel set to an outside bearing.  Roundhouse designed the kit to make sure you didn’t screw up the driver order.  If you are building a six coupled wheel set, you have a problem.  You need to put a flanged wheel set in an opening designed for a blind wheel set.  This photo illustrates the problem.

Mallet 2-6-6-2 Wheels1

The driver won’t even go into the bearing — because the flange is hung up on the frame.  Set this assembly aside until you have accumulated a fair amount of patience.  Then get out your small set of files.  I used a rat tail, a triangle, and a moon shaped file with one flat side.  Then work on the inside of the frame element where the flange contacts the frame.  Figure 30 minutes to do two sets, one for the front wheel set and the other for the rear wheel set.  Resist the temptation to get out your Dremel, your table saw, your chain saw or some other weapon of force.  Keep saying to yourself, “This is an antique part.  If I screw this up, I’m going to have to buy another whole kit to get this part.”  When you are finished cutting the notch where the flange hangs up, you should have something that looks like this.  All three axels should spin freely.

Mallet 2-6-6-2 Wheels 2

Smile.  You just beat one of the more frustrating problems in building this loco.  Yes, I realize my shaft on the blind driver is rusty.  Fixing that problem is nothing relative to what I just fixed.  But notice how nicely polished my bearings are.  That is the result of addressing the second problem in the above list.

We are not ready to put the plate on that holds our drivers in place for two reasons.

  1. The plates have not been modified for the six coupled wheel sets.  We’ll do that next.
  2. This locomotive isn’t going anywhere without gears.  No, I’m not going to attempt to run live steam in HOn3.  We’ll deal with the gears in the drive train section.

Underframe Modifications

All I did here is screw the under frames to the frames and mark the cuts needed to shorten the under frames to match the frames.  While I was doing it I did the same for my 2-4-4-2 frames.  This shot shows the bashed under frames in their correct orientation.

Mallet Underframe1

And this shot shows them laid in place on the undersides of the frame.

Mallet Underframe2

There are two places on the 2-6-6-2 frame and two places on the 2-4-4-2 frame where the end of the underframe is not secured by a cross piece.  See the photo before this one for visual look at the problem.  These are white metal castings.  Those ears without cross pieces are weak.  A small brass strip will be superglued to these pieces across the bottom of the engine strengthen these parts and to keep them in alignment.  I also need to fabricate the piece that connects the front and rear driver blocks and provides a hinge on one end so my frame can articulate.  Finally, holes will need to be drilled and tapped for screws that will secure the underframe to the frame in at least two places.  The steps discussed in this paragraph will be the next steps in frame modifications.

Roundhouse Kit Detail Parts

If I want these engines to be a family, they should have a similar look.  That gets down to paint and lettering.  But it also applies to detail parts.  Here are the detail parts selected for all four engines.

All will have the same stack with the following spark arrestor.  I’ll pick the stack once the spark arrestor arrives.

Spark Arrestor

All will be oil fueled.  An appropriate oil bunker will be added to the tender and to the bunker on x-8-xT engines.  A hatch will be located for oil and another for water.  Here is the water hatch.  The water hatch will be on the tender for engines with tenders and on the bunker for the x-8-xT locomotives.

Water Hatch

This will serve as the oil hatch.

Oil Hatch

All will have the same headlight, mounted on the boiler.  Attempts will be made to electrify the headlights with LEDs,


All will also have tail lights on the rear of the tender for this with tenders and on the back of the bunker for x-8-xT locos.  These will also be electrified.

Tail Light

In some cases, headlights will be mounted on the pilot.

Pilot Mounted Headlight

All will have generators mounted on the top of the boiler.


All will have air compressors mounted on the side of the boiler,

Air Compressor

All will have air tanks mounted below steps on the boiler.  Lines will need to be run from the compressor to the air tank, and from the tank to appropriate places on the locomotive sand tender.

Air Tanks

All will have cylinder lubricators mounted on top of the cylinders.

Cylinder Lubricators

All will have a brake cylinder mounted in a visible area below the tender.  Lines will need to be run to the brake cylinder.

Tend Brake Cylinder

All will have air whistles mounted on the side of the steam dome.

Air Whistle

All will have air ringer bells mounted on top of the boiler.

Ringer Bell

Here is the Pilot Beam for yard switchers.

Pilot Beam

Note: Other detail parts will be added as the project evolves including piping, back head details, tender details, etc.

Backhead Parts – Diagram shows back head for NPC No 21

The problem with back head parts is often, nobody sees them.  But I’m going to list them and show examples.  The following diagram was a collaboration between David Fletcher, Keith Christensen, and yours truly on what the back head might have looked like for North Pacific Coast No 21, my Holy Grail locomotive.  Keith has since passed away and owned the last remaining Class A Climax.  NPC No 21 was unique in that it was the first railroad engine with a marine boiler, and the first cab forward locomotive.  It was also arguably the first locomotive designed from the beginning to be oil fired.  What I’m saying is this back head illustrates parts placement for a very unusual locomotive.


Parts shown on the drawing (along with a few others) are:

  • Throttle
    Backhead Throttle
  • Johnson Bar
  • Gauge Cluster
    Gauge Clusters
  • Brake Stand
    Air Brake Stand
  • Oil Firebox door
    Oil Firebox door
  • Lube Tray
  • Hydrostatic Lubricator – Oil
    Hydrostatic Lubricator
  • Calledan Injector – Water
    Injector with PipingSteam Locomotive Injector
  • Sight Glass
    Dual Sight Glass
  • Tri-Cocks
  • Drip Funnel – Tri-cocks output through the floor
    Drain Funnels
  • Engineer/Fireman Seats
    Engineer-Fireman Seats
  • Auxiliary water pump – Water or oil
    Feedwater Pump
  • Feedwater Heater
    Feedwater Heater
  • Turret – distributes steam to various devices
    Turret Valve

Mallet Specific Parts

  • Pilot Mount Headlight
    Pilot Mount Headlight
  • Unitah Lead Truck
    Unitah Lead Truck
  • Pilot Deck
    Pilot Deck
  • Double Stack
    Double Stack