This is a custom 3 mm driveshaft coupler designed specifically for small-scale RC crawlers (1/12 – 1/24 scale) using 370 motor transmissions.

The coupler connects directly to the transmission output shaft, allowing a front driveshaft to attach securely without modifying the motor or transmission.

Design Notes

• No internal shaft flats by design
  Flats were intentionally avoided due to the small 3 mm shaft diameter, where removing material significantly reduced durability. A full round bore provides greater wall strength and long-term reliability.

• Snap-style transmission interface
  One end of the coupler is sized to work with a lipped transmission output shaft. The lip slightly compresses the plastic internally, creating a snap-on effect that resists pull-off while still allowing minimal movement. This provides better retention than a rigid press fit and reduces stress cracking.

• Reinforced outer ring
  The external ring helps stabilize the coupler on the shaft and improves resistance to deformation under load.

• Twisted geometry for strength
  The twisted body design distributes torque more evenly and reduces the chance of layer-shear failure compared to straight couplers.

Compatibility

• ~3 mm input shaft

• ~3 mm output shaft

• Designed for 1/12–1/24 scale RC crawlers

• Compatible with many drivetrains using 370-size motor transmissions

• Commonly used in WPL-based builds and custom chassis

Printing Recommendations

• Material: PETG (recommended) or PLA+

• Orientation: Print upright (shaft axis vertical)

• Infill: 40–60%

• Perimeters: 4–5 walls

• Supports: not required

Notes

• Bore may be snug depending on printer tolerances; light cleanup may be required

• Designed for low-speed crawler applications

• Not intended for high-RPM or brushless setups

Extras-  feel free to message about getting parts done or any questions.   i am on my engineering journey so all feedback and suggestions would be greatly appreciated :)