Boxer Motor


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.

First Model

Using Lego cylinders and pistons, and their ingenious crankshaft, one can build a very compact little model of an engine with opposing cylinders:

Simple model
fig. 1:  a compact model

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!

Better 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.

a better model
fig. 2:  a better model

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.

close view of the cylinders
fig. 3:  close view of two of the cylinders

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.


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.