Earlier, Sport Pilot wrote:

Most flat fours are not a boxer, and
many twins are not...

...on a non boxer
the pistons on the front and rear pair
will be going opposite directions

First, let me define the terms as I understand them:

The way I understand it, boxer motors are flat opposed engines in which
the connecting rods of opposing cylinder pairs do not share share a rod
journal on the crankshaft. Instead, they connect to the crankshaft at
journals that are spaced 180 degrees from each other. So arranged, each
opposing pair of pistons are both either on the down (power or intake)
stroke or on the up (exhaust or compression) stroke.

Conversely, flat opposed engines in which the connecting rods of
opposing cylinder pairs _do_ share share a rod journal on the
crankshaft are _not_ boxers. And again, that's just the way I
understand it, but a Dogpile or Google search pulls up lots of Web
pages that bear out that understanding.

And by that measure, most flat four motors _are_ boxers. VWs are that
way, and so are Soobs and Lycomings and Continentals. And certainly,
the Ferarri flat 12s are that way, or else the factory probably
wouldn't be calling them "Boxers."

As far as the relative motions of the various pairs of opposing
cylinders goes, I have never heard of that entering into the definition
of "Boxer." I won't say that it doesn't, but I will say that I won't
believe it until I see a credible cite to that effect.

Thanks, and best regards to all

Bob K.
http://www.hpaircraft.com