Also ..I just pulled one motor ...both drives ...had the heads done ..resealed one drive ...serviced the other .....4000.00 ...

how bout for us little guys....1998 350 mpi's turning bravo 1's...currently has approx 175 hours and first 150 hours were fresh water use. it is now being used in 100% salt water some brackish but flushed after every single use...boat usually cruised at between 35-3800 rpm's and when kicked in the a$$ 5000 rpms max @ WOT...whats the life expectency. Synthetic oil changed every 30-40 hours. ?????

ABSOLUTELY. If a manufacturer claims thosuands of hours before rebuild, thats in a Bayliner.

Depends on the engine. The 260 HP 350CID Mercs in my dad's 1983 F272 Formula still check out after over 20 years and 1500 hours. Those mild 350s last a long time. Build a 800 hp engine and you may get 200 hours out of it before a rebuild. Depends on the engine. But, I would think a mild 454 should be able to last a well over 500 hours.

89 formula 260ss 330 B1 all salt water use, oil changed 2/season with 40 weight valvoline racing or castrol oil and not abused - lasted approx. 1300 hrs

My last boat had 400 hp/ 454's. I sold it after 600 hrs and I'm sure it had a season or two left but it was losing performance. It depends how hard you run them. In a cruiser where they never go above 3500 rpm, I could see getting 1500 hrs in a well maintained boat. However in a heavy high perf. boat where the engines are run wide open a lot, 500 hrs is a lot. Also it's a lot cheaper to rebuild an engine before something breaks.

Never seen a high performance motor wear out.The Valve Springs either break or get weak and the Valve touches the Piston .Next thing is the Rod Bolts or Rods stretch and break or touch the Valves(same results, your motor is junk) 500 hrs is the number on a HP motor,although you might get away with just changing the Rods & Valve Springs if you didn't want to do a full rebuild

I agree that after 500 hours on an Hi-Perf marine engine that you had better start thinking of a rebuild before too long. But as far as the connecting rods go, I don't think a connecting rod really needs to be replaced as much as it might need reconditioning and a new set of rod bolts---ARP rod bolts at that. Perhaps that is what Dixie Doug meant.

However, if you are going to go to all the trouble of tearing into the bottom end to remove the connecting rods for reconditioning and rod bolt replacement, then I would say it would probably be a wise idea at that point to just go ahead and rebuild the entire bottom end.

As a marine surveyor in the Ft. Lauderdale area that specializes in performance boats I am asked this question many times. There are so many variables! How was the boat maintained and run? A compression test will tell you if the engine internals are healthy. The 420's had those monster cast-iron manifolds and risers when they first came out and were later changed to the Merc/Gil Offshore type. (Added another 5 hp.). If those engines have good compression and the performance is there, maybe a valve job will be all you need. The big block Chevy bottom end usually only fails if the oil wasn't changed often enough.

Let me say in my post I meant motors with 500-600 hp.Less hp the more hrs you can go,the higher hp less hrs.
KAMMA ,I see you get the ideal