Michael Henry a écrit :

why isn't the Lycoming O-540 or the Continental O-520 an
inverted V?


They are derived from opposed engines, and the manufacturers thought
easier to retain the same cylinders and cylinder heads as their 4
cylinder counterparts.

There have been a number of aircooled inverted engines in the post WWII
period.

But it is hard to make statistics with Lyco/Cont being the sole small
engine manufacturers for decades.

Best regards,
--
Gilles
http://contrails.free.fr