Definition
An hour meter records the total running time of an engine and is the primary basis for hours-based service intervals, but it records only duration and says nothing about the load the engine carried during that time.
Hours drive one half of every service schedule — the other half being calendar time, whichever arrives first. But hours are a measure of duration, not of work or of wear.
Why hours mislead
Two engines with identical readings can be in completely different condition. One may have run at sensible load with documented maintenance; the other may have idled for thousands of hours, wet stacking as it went, or sat unused for years while its coolant and oil aged in place.
Low hours are not automatically good. An engine that has done very little still needs its annual service, and one that has sat neglected may be in worse condition than a well-used one.
What to check alongside
Service records with part numbers, oil analysis trends, cooling and exhaust condition, and evidence of the load pattern. See how to assess a used marine engine.