"Sport Pilot"
groups.com:

Most flat fours are not a boxer, and many twins are not. A boxer
engine will weigh and cost more. It's not really needed on a flat four
anyway, as on a non boxer the pistons on the front and rear pair will
be going opposite directions


Well, i can't thnk of an aviation flat four that isn't, and even the humble
VW is a boxer. So are all Subarus. In fact, I'm hard pressed to think of a
flat four that isn't. maybe the Mucculogh...
As for twins, there were a lot of non boxer opposed twins in the early
days, and they were diabolically vibratory. Much worse even than the
paralell twins that came in the thirties. ( I have a an old Triumph and
will knock your teeth out on a long trip, and it was meticulously balanced
when I rebuilt it)
what you say isn't entirely untrue, but the torsional loads on an engine
with pistons paired on crankpins front and aft would be very high and need
to be stiffened to the point it would be just as heavy if not heavier than
it's boxer counterpart.


Bertie