Constant Mesh 3 Speed Gearbox with Reverse Gear

In this type of gearbox, all the gears of the main shaft are in constant mesh with the corresponding gears of the countershaft. 
Two sliding dog clutches are provided on the main shaft.
Gear ratios: 1.38, 1.00, 0.78, reverse 1.86 and neutral.
Dimensions:  143 x 65 x 100 mm.
Weight: 0.15 kg.
2-speed gearbox: Constant Mesh Two Speed Gearbox by Bootjevaarder | Download free STL model | Printables.com
 

Printing
The parts should be printed on a smooth build plate.
If possible, avoid elephant foot.
I printed the model on a PRUSA MINI, with PLA+ filament, layer height 0.15 mm. 
Glue with Hard Plastic Glue where necessary.
Printing instructions are provided with every .stl file.
 

Assembly 
Assembly in alphabetical order. 
Assembly instructions can be found in 10 screenshots.


The gearbox can be joined to compatible 4-stroke engines
Print parts “N - Sled and Engine Connection.stl”.
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 Sled. 

Compatible 4-stroke engines:
4 Stroke 4 Valves Over Head Camshaft Engine by Bootjevaarder | Download free STL model | Printables.com

4 Stroke Flathead V-Twin Engine by Bootjevaarder | Download free STL model | Printables.com

4 Stroke Flathead Engine by Bootjevaarder | Download free STL model | Printables.com


Operation
Use the cranked handle to rotate the gearbox / engine.
For shifting gears, operate the Sliding Dog Clutches.
To couple / uncouple the gearbox from the engine, operate the clutch lever of the "N - Engine Connection”.