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Old 10-11-2017, 10:29 AM   #21
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They can be rebuilt several times, then there can be some strange issues that might force replacing it. Highly variable there. I would not worry about going in a second time.

Really the rebuild is to replace cylinder kits, almost everything else is in good shape. Some mechs replace other stuff while they are in there, others do a minimum of that.

Since these are 485's, they are high strung for the 71. But if boat was run at trawler speed, they can last near as long as the low rated versions. See if you can parse out of the owner how he ran the boat. 9kts, good. 15kts, not so good.
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Old 10-11-2017, 11:35 AM   #22
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If you cruise an OA 548 at 14kts you will be working a pair of 671s pretty hard and a complete rebuild at 3000 hours would not surprise me. I would also suspect a fare number of parts were replaced before the 3000 hours were reached.

A good rule of thumb is no more than 1 hp per cubic inch. With 671s that would be 6x71=426 hp max.

That said, if you cruise the OA548 at 10 kts those old Detroits would probably go 15,000 hours. On the other hand 671s are probably the cheapest marine diesels to maintain and can be overhauled in the boat.

Now the question is how do you plan to use the boat, how have the engines been used since rebuild, how do you feel about the cost of rebuilding a 671.

I am a 10kt cruiser so I wouldn’t have any issues with 485hp 671s as long as they passes oil analysis.
The DD is a two stroke engine, how does that effect your rule of thumb?
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Old 10-11-2017, 04:04 PM   #23
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Any guidance on how many times they can be rebuilt/overhauled? At 5500 hours the one I'm looking at will most likely be on schedule for its second.
On the tugboat that I work on, we have 4 8-71's aboard. 2 run gensets that run 24/7 alternate every other day. These engines get rebuilt at 30,40,000 hours whether they need it or not. original blocks for at least the last 2 rebuilds. another 8/71 runs a towing machine and the 4 runs a bow thruster. The towing machine engine gets the worst abuse you can do. It gets started up, not usually warmed up and runs for a few minutes and shut down. These engines can take abuse hard service abuse. Yesterdays tech but its also the engine that won world war two.
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Old 10-11-2017, 05:09 PM   #24
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The DD is a two stroke engine, how does that effect your rule of thumb?
That "rule of thumb" started with Detroit 2 strokes.
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Old 10-11-2017, 05:39 PM   #25
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71 series can be rebuilt almost forever. The engine and everything on it was designed to be rebuilt in the field. Parts are available all over the world. Except for catastrophic failure, the block doesn't wear.
DDs are used in every industry. Only in western nations are they being phased out of commercial use. Much of the world doesn't have an EPA and the Detroit engines there will be run until they can't be rebuilt. The demand for parts will be high for years.
I don't like turbo engines. With a bad operator running WOT, you get more cylinder component wear. I have owned both naturals and twin turbo DDs. Keeping out of the high hp rpms (and clean oil) adds hundreds if not thousands of hours of life. Usually you're only talking about a few knots faster. 7000 was the least hours I got out of DD turbos that I ran or had control of. The most hours would sound like a sea story.
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