In this type of gearbox, all the gears of the main shaft are in constant mesh with the corresponding gears of the countershaft. A sliding dog clutch is provided on the main shaft.
Gear ratios 0.75, 1.33 and neutral.
This model was designed in Tinkercad.
3-speed gearbox with reverse gear : Constant Mesh 3 Speed Gearbox with Reverse Gear by Bootjevaarder | Download free STL model | Printables.com

Printing and Assembly
The parts should be printed on a smooth build plate, not textured.
If possible, avoid elephant foot.

I printed the model on a PRUSA MINI, with PLA+ filament, layer height 0.15 mm.
I use Hard Plastic Glue where necessary.

For assembled and exploded views, use .stl files A0… X0… (do not print).
Printing and assembly in alphabetical order (A1 → X1).

Choise between Spur and Herringbone gears (A4 B2).

Dimensions: 95 x 63 x 101 mm.
Weight: 0.1 kg.

Join gearbox to “4 Stroke Flathead Engine”
Print parts X1 → X5.
4 Stroke Flathead Engine v1.0 by Bootjevaarder - MakerWorld

To couple the gearbox to the engine, the “Cranked Handle” of the engine must be removed.

The engine, coupling bracket and gearbox click into the “X1-Sled”.
Important: Part “X2-Coupling Bracket” must be assembled in the correct orientation (ARROW).


Operation
Use the cranked handle to rotate the gearbox / engine.
To couple / uncouple the gearbox from the engine, use “X5-Coupling Selector Fork”.
For shifting gears, use “A3-Selector Fork”.