What are high hours for an engine?

The rule of thumb here is that every 50 hours of marine use is approximately equal to 3000
miles of automobile travel.  Today’s cars get 120-150,00 miles out of their no-lead gasoline
motors, so 2000-2500 hours is about max for gasoline marine motors and outboards.  Diesels
are more rugged, run at lower RPM and 6000-8000 hours is the high end before rebuild. Age is
also a factor especially if salt water cooled. Older, low hour engines are not necessarily a
bargain as lack of use can present another set of problems. Here in the northeast, 50 hours of
use per year is considered the norm. Always ask for maintenance records.
From the Surveyor's Notebook