Atlas PRR H21A Quad Hopper Bolster & Coupler Conversion
Correcting the underframe of that really nice ready to run Atlas O car and replacing the Atlas trucks with more detailed models, such as Protocraft, requires a little 3D printed bolster part that perfectly fits the Atlas underbody. All forces that occur within a long train are being transferred into the injection cast plastic parts of the model, not into the 3D resin parts.
The new part is available in combination with a much nicer looking draft gear box to accomodate a Kadee or Protocraft coupler. For my personal two models I use Rich Yoder 70t Andrews trucks. Please read the instructions below.
Here you can see the new bolster fully installed under the car.
An earlier test version sitting on the Atlas underframe.
Instructions
- Cut the plastic brake line at the bolster position. Unscrew the Atlas bolster parts and carefully disassemble them. Take care not to break the brake cylinder on the B-end.
- Sand down the plastik bolster as shown in the picture. You want the bolster to be level with the coupler box mounting surface. Leave the king pin cone as shown. I use a belt sander for the job and then celan the parts with a fine file, especially around the king pin cone.
- Test fit the printed part over the plastic bolster. Make sure there’s not gap between the Atlas part and your new draft gear box. You will find the matching screw holes for the coupler box. Tap these holes on the Atlas bolster part using an M2 tap.
- Tap the king pin cones according to your trucks. Since the opening is already a bit large, I chose M2.5 for my Rich Yoder trucks.
- Test fit your trucks. The printed bolster has two prominent side bearings that I designed for the Yoder 70t Andrews trucks I use. Should they interfere with free movement, you might need to sand them down a bit. You want the truck to sit on the center glide pad and not on the side bearings. Check that before mounting any parts in place!
- Prepare the Kadee coupler by filing off the tail dimple. The Atlas holes are narrowly spaced, in order to fit the Kadee inside, the dimple has to be removed. Load the spring and insert the part carefully into the draft gear box.
- Secure the coupler with two M2x10 screws. The 10mm long screws are a perfect fit, you can not use longer screws since they would run into other parts.
- Fit your new bolster sandwich in place, be careful with the detailed B-end. The two end screws can be re-used but the two M2.5 screws that go into the bolster need to be longer. I recommend M2.5x10mm for the task.
- Attach the rest of the air line and hide it behind the bolster.
Bill of Materials
M2x10 Screw (four)
M2.5×10 Screws (four)
M2 Tap
M2.5 Tap
Pin Vise for tapping
The new darft gear box replaces the Atlas coupler box and is designed to work with Kadee and Protocraft couplers. Both types fit right in but need a different cover part that comes with the download package. The striker plate represents my version of the original configuration on the prototype freight car.
Atlas H21A Hopper Bolster Parts
Files incude a full bolster and draft gear combination, as well as both components as separate parts. You also find the correct coupler covers for Kadee and Protocraft couplers.
