Is there a general rule for how many hours an engine can have on it before it should be rebuilt? looking at buying a 55 Sedan Bridge and a lot of the boats I am looking at have 1000 hours on the engines. Any input would be great.
Also, what about the generator? How many hours before rebuild?
That is totally determined by the pervious maintenance history and use. An abused boat that has been overheated, not maintained and run in an overloaded condition might need an early rebuild. That is why you nearly always end up here with a recommendation to do a mechanical survey and fluid analysis on any used diesel boat.
Properly cared for, diesels can last longer than you want to keep the boat. A friend of mine, who owns a charter boat, bought a pair of 3196's that Caterpillar rebuilt twice after 2 aftercooler failures for a Sea Ray 480DB owner in our marina. Sea Ray didn't want to ask him to take rebuilt engines a second time and they just bought him 2 new 3196's and Cat sold the rebuilt ones to my friend. That was in 2000 and he had 14,000 hours on the pair in June when I saw him at the fuel dock. He has done routine maintenance, replaced a couple of alternators, a few injectors and one starter, but has not missed a single charter in 12 years.
Generators are designed to sit there and run at relatively low speeds. Seeing them with several thousand hours on them isn't unusual at all. However, comparing engine time to generator time may give you a clue as to how the boat was used by previous owners.
Guys, I think the op stated it's getting a bit off topic. I understand why it would go from the Cats to other items, as the issues pertain to a good survey. I will keep this brief as the OP knows I'm...