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In one of our other endeavors we discovered that old time HO 4-4-0s match early 1870s locos like this Porter-Bell. This is the New Tooling HO Bachmann.
Early 1870's locos were really small, standard and narrow gauge.
In an earlier venture we built a cab from styrene to develop an Sn3.5 from an HO model. Now there is 3d printing that seemed to be an easier direction especially since I was a 3d designer in a former life.
Found this cab listed as "1/55 CK Holliday Disney" cab which was close to the right size.
The cab comes as a flat.  Fit the front first. Then assemble the cab.

The material is brittle, don't press the part on to the loco body as it is difficult to pull off.

The cab front needs to be fitted to the loco body. The opening at the arrow must be enlarged.

Place the HO cab onto the new cab and trace the opening and enlarge the cutout.

Use a jig to make sure the cab body is square, D-oh!

The parts should be placed in warm water and flattened.

Filled the cab roof hole by gluing a .010 styrene piece on the interior and filling the hole with .060 styrene.

The styrene is sanded to the cab roof shape

To remove the boiler there are two screws under the cab. Pull out the boiler braces at the front. Lift up the boiler at back and push it forward. There is a lug at the front.
Remove the cab by cutting the lugs and wiggling it off.

Push wires down through the slot at "A". Tied the wires together with thread. Note which terminal the blue wire goes to on the headlight.
To protect the sanding lines place on a solder or tape tube.
That is one of the domes from the cab maker.
Cut off the cab mounting lugs.
This is what a 1870's cab end looks like.
Roughed in a cab mounting lug.
The lugs came out to be 3/16x3/16. Used 1/8 thick styrene.
Acc'd the mounting lugs hoping to square up the cab end. Worked on one side and not the other.

Found out later that the 3d acrylic can be reformed by warming.

Place the cab on the boiler and drill up through the original mounting holes with a #00-90 tap drill.

Enlarge the holes in the boiler with a #00-90 clearance drill.

Screw the original mounting screws into the mounting holes.

Mount the cab on the boiler and take a smoke.
Sand the back of boiler flat. Cut the red area out. Didn't do it originally but found that was the best course.
Glue .030 thick pieces of styrene to the boiler piece but not to the cab. These are an extension of the walkway.
Scaled the end view and glued the piece to a piece of .060 styrene.

Cut out the  relief with a jeweler's saw.

Glued the piece to the boiler and after it was stuck noticed that the piece wasn't right.
Looking at the etching scan the original is off kilter, Arrrgh.

Clever filing would solve the problem.

Later the piece fell off so could have made a new one.

The .060 was too thin for the end piece. Glued .060 fillers. Should have cut back to the red lines originally.
Dill hole to clear .020 wire.

Don't try to drill in to the upper area at the arrow.

There are two styles one doesn't have those little steps.
Shape the end piece and   walkway extensions.

Glue the wire pieces.

Put the cab on and run a train, wheeeeeeee! Can smell layout coming.
There was a You-Tube video about painting locos with Black Gesso.
Painted the cab black so  trains could be run while cars are being built.

Later we will return to detail the loco and paint the loco properly.

Our world at the PcalRwy were early adapters of Janney coupler. The theory is the railway got early prototypes to test.

The Proto HO: Accumates are the right size.

The coupler sticks out too far because the loco had a previous life in a beta existence.

The Bachmann HO boxer glove coupers can be easily replaced with couplers more to scale.

These are McHenry's.

Putting the loco back together created an Eeeeeeek! moment. The cab was askew. Check everything the problem was overtightening of the mounting screws for the cab weights and cab.
The Tender Electrical Pickups for the Bachmann 4-4-0 are shaky. Click here to correct that.
Click here for a PDF version
That is getting the loco in service we will have additional updates on detailing and painting.
Not satisfied with the flat cab, developed one in 3d CAD with the same dimensions in a couple of hours. Had a cab printed at Shapeways and two week later had my cab. A lot easier than scratchbuilding