I have a merc 420 hp with 110 hours on it. Tonight I did a 20 mile (each way) cruise. On the way there I ran 3200-3500rpm. One the way home I ran 4000-4200rpm. The boat really comes alive at 4000rpm. Is it safe to run it that long at that rpm?

as long as your oil&water temp looks good i say go for it.

Water got up to 160. I don't have an oil temp guage. It has a cooler on the Side of the motor that's about the length of the block, 1/3 power steering, 2/3 oil cooler. I probably should get an oil temp guage.

I believe recommended wide open throttle range on the 420HP engine was 5000-5200rpm.

I used to have those engines. I ran them all day long at 4000RPM, and quite often at 5000rpm for several minutes at a time.

Thats a old motor to have 100 hours on it . But i would love to have it

It has 100 hours on the rebuild