A flathead engine, also known as a sidevalve engine is an internal combustion engine with its valves contained within the engine block, instead of in the cylinder head, as in an overhead valve engine.
The boxer configuration with two opposite cylinders is/was used by motorcycle manufacterers as BMW, Douglas, Harley-Davidson, Indian, Lilac, Zündapp, among others.
The boxer engine is commonly used in aircraft applications because they generate a lot of power from a light engine.


Printing and Assembly
The parts should be printed on a smooth build plate.
Avoid elephant foot and over-extrusion.

I printed the model on a PRUSA MINI, with PLA+ filament, layer height 0.15 mm.
Use glue where necessary.

For assembled and exploded views, have a look at the .stl files in the “VIEWS” map.
Printing and assembly in alphabetical order (A1 → F2).
In the “OPTIONAL” map you find 3 .stl-files for your own preference.
Printing tips are available for many parts.

Dimension: 125 x 142 x 112 mm.
Weight: 0.32 kg.


Metal parts
Four coil springs are needed for the valves, 10 mm width x 15 mm height. Wire thickness 0.6 or 0.7 mm (AliExpress).


Compatible Gearbox
For joining the gearbox Constant Mesh Two Speed Gearbox by Bootjevaarder - MakerWorld, print “OPTIONAL - Gearbox Stand.stl” instead of “B5 - Stand.stl”.