The 6 cylinder boxer configuration is used by manufacturers like Porsche, Subaru and Lycoming (aircraft).
The model comes with moving crankshaft and pistons. The crankcase cap is removable.
Dimension: 96 x 143 x 61 mm.
Weight:  0.14 kg.

Printing
The parts should preferably be printed on a smooth build plate.
Avoid elephant foot and over-extrusion.
I printed the model with PLA+ filament, layer height 0.16 mm. 
Printing and assembly in alphabetical order (A → K).

Assembly 
The crankshaft cheeks are numbered from 1 -12.  
The arrows on the outside of the cheeks should be aligned and point in the same direction.
Place conrod to 2. Glue 2 to 1. Bolt 3 to 2 with 2 bolts.
Place conrod to 3. Glue 4 to 3. Bolt 5 to 4 with 2 bolts.
Place conrod to 5. Glue 6 to 5. Bolt 7 to 6 with 2 bolts.
Place conrod to 7. Glue 8 to 7. Bolt 9 to 8 with 2 bolts.
Place conrod to 9. Glue 10 to 9. Bolt 11 to 10 with 2 bolts.
Place conrod to 11. Glue 12 to 11.

Now place the assembled crankshaft in the Crankcase, and close the Crankcase by glueing the Crankcase Lid to the Crankcase.
Glue the Crankcase Feet to the Crankcase.
Glue the Turning Wheel to either end of the crankshaft.
Finally place the Pistons to the Conrods using the Piston Pins, and click the Pistons into the Crankcase cylinders.