In last week’s post I started showing you how my new N Scale project is coming on, Alco’s C-855.  And as promised in this week’s post I am going to show you how I intend to lengthen the donor chassis for this locomotive.

The chosen chassis, as modeled below, for the C-855 is going to be Con-Cor’s 4500 Gas Turbine/GE U50 chassis.

Although the chassis has the right trucks etc for the C-855 they are at the wrong centers and need to be spread apart by 10mm.  That doesn’t sound a lot but in N scale it can make a huge difference visually, it’s 1.6 meters (5′ 3″) on the real locomotive.

I looked at several ways to do this and each time found a reason why it couldn’t be done that way.  The main issue is the motor; it is centrally positioned in the chassis with two drive shafts powering the two furthest sets of trucks.  The two inner ones are not powered.  Spreading the trucks will also mean the drive shafts will no longer reach them. And if only one end is lengthened it may cause an unbalanced load to be applied to the motor.  The motor hangs down in the frame and on the 4500 Gas Turbine/GE U50 this is concealed by a fuel tank mounted between the trucks.  On a side note, the 4500 Gas Turbine didn’t have a fuel tank but a battery box in this location and I already offer a 3D printed replacement part to correct the 4500 Gas Turbine which you can find here.

The C-855 side frames hang down in this location which is a visual aspect quite unique to the C-855.  You can see this on the shell render below.  Again if the donor chassis was only extended at one end the areas where the motor hangs down would be visible on one side of the frame.

Therefore my only choice is to leave the motor in the center of the chassis and extend it in both directions.  Because the chassis provides the strength and weight for the locomotive I need to do this in such a way that it doesn’t compromise the chassis.  Also I want to make it easy to do.

I decided to totally remove the center section which holds the motor in place and replace it with a longer 3D printed part.  Below you can see a rendering of the chassis showing were I plan to remove the section.

And then below is a rendering showing the main parts in the correct place.

The new section in the middle will actually need to be two parts, as pictured below.  This is because the top of the chassis conducts electricity from one rail and the bottom from the other.  Also adding more material into the 3D print adds unnecessary cost.

The two new parts have been drawn to match the sections which have been removed so they will clamp the motor in the right place, as you can see in the rendering below.  The new parts will be printed in stainless steel which will maintain the weight required by this locomotive.  I will also offer the extension pieces in plastic as a cheaper alternative.  Where I plan to cut the chassis will form a natural step, making the joint to the new sections stronger.

The last part of the puzzle are the drive shafts which will now be equally 5mm too short.  To resolve this I will be 3D printing an extension piece for both.  Below is a rendering of one of the standard drive shafts.  The circular cup gear on the end fits over the drive gear on the motor.

To extend this I have simply designed a part that will fit into this cup gear with the same configuration on the outside as shown below.

The new part will be glued into the drive shaft completing the extension.

With the whole thing assembled the lengthened chassis should look like this.

One more modification to the chassis will also need to be made at the cab end so that it fits into the C-855 shell.  But until I finish the design work for the shell I won’t know the extent of the modification, so I will share that with you once I have 3D printed the new parts and have a real C-855 chassis to show you.