Monday, 11 January 2021

AB Locomotive Assembly Instructions


AB Loco Assembly Tips - Draft: More Details to be Added.

These instructions are preliminary and further information will be added as time permits.  Please do not hesitate to contact Marbelup Models if you find any missing information as this will prompt updates to this page.  

Parts List 

3D-Printed Parts

  • Underframe
  • Fuel Tank
  • Body - cab and short hood
  • Body - long hood
  • Parts sprue containing motor mounts, drive shafts, and coupler shims.
  • Bogie Sideframes - not available from Marbelup Models currently.  See text below for options.

Mechanical Parts

  • # North Yard (NZ) drive bogies for WAGR A Class with 2 mm diameter worm shafts.
  • # Motor - Auscision part no. SP-1.  Includes twin brass flywheels.
  • # Drive cups to suit 2 mm diameter shaft. (2 required).  Four drive cups are included in North West Short Line (NWSL) U-Joint set 489-6.
  • # Brass Strip 6 x 1 mm, 2 required.  ( K&S part 9843 contains 3 strips.)
  • Fixing screws for body, couplers and fuel tank:
    # 2-56 x 11 mm, 10 required for body fixing screws and couplers
    # 2-56 x 12 mm, 2 required for fuel tank.
    Kadee #256 nylon screws can be used for the fuel tank, couplers and end body fixing screws.
  • # Fixing screws for motor mounts - self tappers, 1.5 mm dia. x 6 mm long (4 required). 
For items above marked with #, Marbelup Models has a limited quantity available for sale.  

Miscellaneous Parts

  • # Air hoses - e.g. Detail Associates #6206 - 12 required.  (Vacuum brake hoses are included.)
  • # Handrail Knobs (4) for uncoupling levers - e.g. Markits 1.5 mm (short) - available from Railwest Models.
  • # Handrail Stanchions (19) (optional) - A-Line D1070 or D1071. .
  • Brass wire for handrails, etc. (0.4 mm, 0.6 mm - 3 x 300 mm lengths)
  • DCC decoder and speaker (both optional)
  • Light Emitting Diodes (LEDs), optional, for headlights 4 x 3 mm (sunny white) and optic fibre (2 mm)
  • LEDs (red and white) and optic fibre, if operating marker lights desired
  • lead or other weighting material
  • Kadee couplers #146 for scale coupler height,  #149 for HO coupler height (see below)
  • Kadee draft gear boxes #242 (supplied as standard with Whisker couplers)
  • Paint and decals, as desired.
For items above marked with #, Marbelup Models has a limited quantity available for sale.  Other items will need to be purchased from local hobby shops or online sources.


Remove the support structure from the 3D-printed underframe.  

Take particular care around areas with delicate detail, including the vacuum hoses and the areas highlighted in the following images.

Remove and save the four horns printed at the front of the central motor cutout, leaving the spigots attached to the horns intact as much as possible.  Only two are required, with two spares.

Underframe Details - Right Side, Front

Underframe Details, Right Side, Rear

Underframe Details, Left Side, Front

Underframe Details - Left Side, Rear

Clean out the indicated holes with a 1.8 mm drill bit and tap with 2-56 thread.  

Underframe Holes to be Tapped 2-56 (viewed from below)

Clean out the indicated holes with a 1.2 mm drill bit for later fitment of the motor mounts.  

Underframe Holes to be Drilled 1.5 mm (viewed from above)

Brass Strips 

The underframe is reinforced by two brass strips, which must be cut to length and notched, as the diagram below, to fit snugly behind the end steps.

Dimensions for Brass Strips (2 required)

Brass Strip Placement

Secure the brass strips, e.g. with superglue.

Bogies and Drive Train

Remove and discard the brass cross-bearers from the North Yard drive bogies.

Fit a drive cup to the worm shaft on each bogie, so that the end of the shaft is flush with the inside of the drive cup.

Install the bogies using the shouldered brass screws and springs provided, with the spring on top of the 3D printed underframe.  

Note that it will be necessary to temporarily remove two of the tiny black screws securing the inner bearing for the worm shaft, to allow the work shaft assembly to pivot upwards during the installation.  When replacing them, do not overtighten.  The worm drive will run smoother if these are backed off approx. 1/2 turn from tight.

Fit the two 3D-printed motor mount brackets using four 1.5 x 6 mm self tapping screws.

Fit the two 3D-printed drive shafts into the drive cups in both the motor and bogie work shafts, and gently guide the motor into the mounting brackets.  Insert the motor slightly angle to the vertical at first, to allow the lower solder terminal to fit around the mounting bracket, then rotate the motor to the vertical position as it move into its proper position.

Mechanism Assembly

Suitable bogie side frames are available from Railwest Models in cast resin, or as 3D-printed parts designed by Butlers Swamp Models, which may also be available from Westrail Models.  

Butlers Swamp Models has kindly made the 3D design for their bogies available, and Marbelup Models intends to produce a revised version of the 3D-printed bogies with a couple of cosmetic improvements when time permits.


The coupler height of the AB loco has been set at the correct scale height for WAGR/Westrail narrow gauge rollingstock, i.e. 12.2 mm (above rail height) rather than the HO coupler height (9.9 mm) which is commonly used for most Sn3½ models.  This has been done because the correct height "looks better" for modern locos and rollingstock.  For correct scale coupler height, use Kadee #146 couplers.  

Provision has been made in the underframe design to lower the couplers to match the HO coupler height by using Kadee #149 couplers which have an "overset" shank together with a spacer approx. 1 mm thick between the coupler box and mounting pad.  

If intending to fit couplers at HO height, file out 1 mm from the bottom of the coupler mounting holes, as per diagram below.  The headstock is only 0.5 mm thick in this region.  Up to 1.25 mm can be removed, if required, to accommodate fine adjustment of coupler height.

Headstock Modification for HO Coupler Height

The #146 and #149 couplers are both "long shank" style which avoids the problem of the "glad hand" on opposing couplers from fouling the cowcatcher.  The suggested draft gear boxes are Kadee #242 (supplied as standard with Whisker couplers).  Kadee suggest assembling the coupler so that the main part of the draft gear box is on the bottom and the lid is on top.  The draft gear boxes simply snap together.  

DCC and Sound

Various sound decoders can be used provided they feature the EMD 645 Non-Turbo Prime Mover sounds.  

The AB fuel tank is specially designed to accept an Auscision SP-58 speaker.  These are available from Auscision or their Australian Modeller Shop as a spare part.  The mounting tabs on the speaker should be tapped 2-56 and countersunk screws, e.g. 2-56 x 8 mm, inserted from the underside of the fuel tank. 

Provision has been made for automatically connecting the speaker wires when the fuel tank is installed.  If you wish to pursue this option, contact Marbelup Models for further information on the parts required.

Body Preparation and Details

Take care when removing the support structure from the body, especially around the cab windows to avoid damaging the vertical dividers between the windows.  

Once the body has been "cleaned up", it is also necessary to remove the two temporary braces which link the fixing posts near the centre of the body, as highlighted in yellow below.  These have been included to protect the body during production and shipping, and must be cut away to provide clearance for the motor, etc.  These can be cut with a fine-toothed razor saw or a cutting disk in a rotary tool, at low speed.  

The eight fixing holes in the body shell and nose should be tapped with a 2-56 thread.  If necessary, clean out the holes prior to tapping using a 1.8 mm drill bit.

Use of A-Line Handrail Stanchions

An alternative, which is potentially easier than soldering, is to use preformed steel handrail stanchions available from A-Line (Division of Proto-Power West, USA).  These are available from various online hobby shops and/or eBay.  (Marbelup Models has limited stocks of these stanchions available at $15 per pack of 35 stanchions.  Each loco requires 19 stanchions.)

A-Line Handrail Stanchions
Although these stanchions are intended for HO locos, the two longest sizes are sufficiently long to be trimmed to the correct length for the AB .  The A-line part numbers are D1070 (extra long) and D1071 (long).  The stanchions should be trimmed at the bottom.  After trimming, file off any rough edges from the cut end.  

Note that the length should be measured from the top of the "loop".  When installed, the bottom of each stanchion should be flush with the bottom of the running boards.

The holes in the 3D-printed chassis should be carefully drilled out to accommodate the stanchions (suggested drill size: 0.75 mm  - test on some scrap plastic first).  

After preforming the long handrails as per the PDF template, test fit each handrail without stanchions.  Once satisfied with the fit, remove each handrail and thread on the stanchions.  It is suggested to orient all stanchions the same way, e.g. with the open side of the top loop inwards.  

Carefully insert each stanchion vertically into the drilled holes.    It is probably best to insert the stanchions part way initially, then go along and press them down to the full depth, and also insert the ends of the handrails into the holes in the underframe.  Make sure the bottom of each stanchion is level with the underside of the running boards.

Once all the stanchions are in place and the long handrails threaded through, check that all the stanchions are vertical and make any necessary adjustments.  Then, the stanchions can be glued into the holes in the running boards and the handrails glued into the holes in the underframe.  It is optional whether to glue the handrails where they pass through the loops in the stanchions.

Separately form the U-shaped handrails on the side, and the end handrails from 0.6 mm wire, as per the template.  Install and glue the U-shaped handrails into the underframe.

Temporarily fit the loco body to the underframe in order to test fit the end handrails, the top ends of which locate into holes in the loco body.  Only glue the bottom end of the end handrails, to allow for future removal of the body.  It is suggested that the top end of the handrails should protrude approx. 1 mm into the body, to allow them to be sprung out slightly when the body is removed.


Four horns are included as part of the 3D-printed underframe.  Carefully remove them from the underframe, leaving the spigots attached to the horns intact as much as possible.  Only two are required, with two spares.

(Alternate horns can be source from detail parts suppliers e.g. in brass, if greater robustness is desired.)

Starter holes are provided for the horns on the front wall of the cab (right side) and on the left side of the long hood, just behind the vestibule door.  Consult photos appropriate to the era being modelled to determine correct horn placement.

Drill out the starter holes to suit the diameter of the spigot on the back end of the horns.  Carefully trim the unwanted spigot (projecting sideways or lengthways) from the horns.


The holes provided in the body shell for headlights are nominally 1.8 mm diameter, but typically slightly undersize due to the 3D printing process.  The headlights should be 2 mm in diameter, so drill the holes out to 2 mm taking care not damage the thin surround around the hole.

(The holes are deliberately undersize because the thin surround would not print otherwise, as the minimum detail thickness is 0.5 mm. and the outside diameter of the surround is 2.8 mm.)

One method of lighting the headlights is to use a short length of optic fibre together with a 3 mm LED (Light Emitting Diode).

The AB locos had "sealed beam" headlights for which "sunny white" LEDs are a suitable approximation.  (Many older locos had larger diameter headlights with replaceable bulbs which tended to have a more yellow appearance, similar to "golden yellow" LEDs.)

Note: Incandescent (filament) lamps are not recommended due to their high operating temperature which may damage the 3D print material.

To attach the LED to the optic fibre, 3 mm black heatshink tubing is recommended.  With the use of a hot air gun, the heatshrink tubing will shrink to form a snug fit over the optic fibre.  However, the hot air may well be hot enough to melt the plastic optic fibre, so the trick is to shrink the tubing over the shank of a 2 mm drill bit, them slide the drill bit out and slide in the optic fibre.

Typical steps are:

  1. Cut heatshrink tubing to length, e.g. 8-10 mm.
  2. Hold 3 mm LED in a small vice by its legs, with the clear lens uppermost.
  3. Push the cut length of heatshrink tubing over the clear lens of the LED.
  4. Hold the shank of a 2 mm drill bit inside the heatshrink tubing while applying heat from a hot air gun.
  5. When it has cooled, withdraw the drill bit and insert the desired length of optic fibre.  For the rear of the loco, you will probably have to feed the non-flared end of the optic fibre through the curved hole provided (from the outside) and attached the LED with attached heatshrink tubing on the inside.
CAUTION: Do not used the hot air gun or other heat source to shrink the heatshrink tubing in close proximity to the loco body or other 3D printed parts as they may distort due to the heat.

For the dual headlights, it is simplest to wire the two LED in series as this reduces the wiring and avoids the need for two separate resistors.  A resistor of around 2K2 (2,200 ohms) is a suggested starting point for LEDs powered from DCC decoders.

Marker lights can also be drilled out and illuminated if desired.

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