This Boxer Air Engine is a modular flat-four style pneumatic engine inspired
by the classic boxer layout, where opposing pistons move in opposite
directions for smooth, balanced motion.

The full assembly is built from 4 separate single-cylinder air engines, and
each one can run on its own. This lets you print, build, and test each
cylinder module individually before installing them in the shared frame and
combining them into the complete boxer engine.

Each single-cylinder module is based on my previous single-cylinder engine
concept, but with an updated crankshaft design:
- Square front shaft
- Square rear socket (hole)

These features allow the crankshafts to connect end-to-end so all four
cylinders can be phased correctly in the final boxer setup.

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I printed mine on a Bambu Lab A1 mini using PLA.

I used a little baby oil for lubrication. Some sanding or light filing is to
be expected, but usually not very much.

All moving parts must move very easily for the engines to work correctly.

This engine can run from a balloon, an air duster, or an air compressor.

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- Mark each crankshaft and flywheel with a marker before final assembly.
- Align timing so Cylinders 1 and 2 are on their power strokes at the same
 time.
- Align Cylinders 3 and 4 the same way with each other, but opposite phase to
 Cylinders 1 and 2.

This staged approach makes troubleshooting easier and helps ensure a
smooth-running final engine.

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- Modular design: 4 independent single-cylinder engines combine into one boxer
 engine.
- Each cylinder can be tested individually before full assembly.
- Interlocking square crankshaft ends simplify mechanical coupling.
- Correct phase alignment is critical for balanced boxer operation.
- Marking parts helps maintain timing during final build.