A boxer motor is a piston engine with opposing cylinders. The best known example is the engine of the historic Volkswagen Beetle, but the design is also popular in light aircraft.
"Boxer" refers to the fact that a pair of pistons is arranged such that they move inwards (towards the crankshaft) together and therefore seem to "box" each other.
Using Lego cylinders and pistons, and their ingenious crankshaft, one can build a very compact little model of an engine with opposing cylinders:
This runs fine and looks very good. But it is also clear that the pistons do not "box" each other: each pair moves as a pair: both go left or both go right at the same time.
This configuration, if used for a real engine, would cause severe vibrations, and because one pair of pistons moves left when the other moves right, it would also cause severe torque around a vertical axis.
This is not a good model!
We can make a better model, but…
The price for a better model is to make two pieces that do not exist as standard parts. They are a piston rod washer, of which one is needed for each piston rod, and a quarter bushing, of which quite a lot must be made: I needed 20 in total.
Quarter bushings are fairly easy to make by carefully cutting a half bushing into two. A very fine jigsaw is a good tool to do this.
Piston rod washers unfortunately are much more difficult to make: I had to use a lathe to turn two washers to the right diameter, starting from quarter bushings. The washers are needed because each piston rod is now independent, whereas in the normal Lego usage two rods form a matching pair.
It is possible to make the better model with only standard parts, but then the construction becomes rather large and the cylinders begin to look too small compared to the rest.
This version of a boxer engine does in fact correspond to a real engine as used today in light aircraft, including the air cooling, the boxing pistons, the offsets of the cylinders within a pair.
If a thin version of the 1×2 liftarm existed it would be possible to make the good model even more compact and save a number of quarter bushings.
Obviously, whether or not a model is satisfactory depends on the aspects one wants to model: the simple compact version is good enough if we want only to show opposing cylinders, but if we want to show boxing it fails.
The large number of quarter bushings is the result of minimising the offset between opposing cylinders. First, to put the piston rods as close as possible to the crank (black short liftarm in the middle of fig. 3), the rods must be oriented so that their bearing faces away from the crank. That leaves an unsatisfying space between the rods and the black crank, which I filled with the washers. The washers are not really washers as they are fixed to the axle, but that does not matter. Second, the piston rods and washers are fitted to a Lego crank, but this has a protruding axle piece of 12mm. Of those, 4 go into the black centre crank, and another 6 are taken by the piston rod bearing + washer. Thus we are left with a space of 2mm, where I used a quarter bushing.
The centre crank is not supported, which is mechanically not very good but does work for small engines. The next set of two pistons form an identical assembly, but between the two sets there should be a fixed bearing to support the entire crankshaft in its middle. There are three crankshaft bearings visible in fig. 2: one at either end and the one in the middle.
The position of the pistons on the crankshaft determines the position of the cylinders and the bearings of the crankshaft determine the rest, from the two follows that quarter bushings are needed and where they should go.
Cutting up 10 half bushings to make 20 quarter bushings is something that might be frowned upon, but if it is admitted, then there is an alternative way by modifying four other pieces instead and getting an even more compact model: if you cut 2mm off the protruding axle piece of each of the four cranks, then all quarter bushings can be dispensed with and the entire model becomes 8mm shorter.