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Old 10-14-2009, 02:51 PM
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Default Detroit 671's opinions

I am looking at a boat that has 1984 detroit 671's in it and they have 4000hrs on them. what sould i do. Should I not even consider buying the boat with out expecting to overhaul them. Any advice is appreciated

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Old 10-14-2009, 03:03 PM
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what model 671
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Old 10-14-2009, 03:04 PM
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Are they 6-71N's? (normally aspirated)if so, I have seen them with 10000 hours, are they JT's? The higher the power rating, the fewer the hours of reliable performance. I have heard some downright scary TBO times on the high horsepower models.
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Old 10-14-2009, 03:04 PM
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Most Detroit guys will tell you that 4000 hours is past re-build time if they are high HP models. There are so many variables that go into that equation though that it truly can't be exactly predicted. The HP is the biggest factor as already stated, a 6-71 NA may last longer than you but a 450HP model, not so long. One thing for sure though, understand the cost and account for it in your offer.
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Old 10-14-2009, 03:07 PM
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It is the 450hp version
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Old 10-14-2009, 03:33 PM
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Any high HP diesel is due for a rebuild at 4000 hours. If the hour meters are mechanical tachometers/hour meters the hours are probably even higher. These meters count hours at 1800 rpm. That means every 3 minutes at idle counts as one minute on the hour meter. Chances are they may have already been rebuilt. If so at how many hours and how much was done. Also find out the "marine age" of the heat exchangers. These (again on any diesel) are usually only good for 7-10 years in salt water. Also mixers. If you are looking to buy and then rebuild find out if they are Johnson & Towers. They sell long blocks for around $9,000. Still have to use all your marine gear. If you get the serial numbers you can call them and get exact numbers for your engines. All that said two quick things to check. 1) Start the engines cold and time how long it takes for the smoke to stop. 2) Run the boat up on plane and verify no black smoke and RPM 2500+ If it passes these tests the engines are probably ok. I'm on my fourth year with mine.

PS. if you are coming from gas engines the first oil change at 24 quarts per side will be a bit of a shock. CF-2 only. NOT Delo-400
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Old 10-14-2009, 03:56 PM
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How long should it take for the blk smoke to go away. Wouldnt be cheaper just to do an overhaul on the engines
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Old 10-14-2009, 04:04 PM
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Quote:
Originally Posted by Oh Reely! View Post
How long should it take for the blk smoke to go away. Wouldnt be cheaper just to do an overhaul on the engines


What type of boat? Depends on what they need. Might be able to do an in boat rebuild. But, if they have to come out, exchange for the JT's may be the way. As stated above, you need more info. How old is all the marine gear? Time since overhaul (cuz I can almost guarantee they've been majored, if they have'nt, you will know when you run them) Aslo, there is wealth of info at boat diesel dot com. With that said, the 6-71 is an ancient reliable design. Big and heavy. The naturals were just bomb proof. I saw some 871 N's that the operator swore had 20K plus hours on them. Sounded like bolts in a saucepan, but running. 2 stroke power !!!

And like Battlewagon said, 6 GALLONS of oil. LOL, I forgot all that fun!
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Old 10-14-2009, 04:07 PM
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Just talked to the original owner it is a 38 topaz and he says all the gear is up to date but the motors have not been overhauled at all.
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Old 10-14-2009, 04:12 PM
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had a friend with 12-71 nat. that engine sounded like a p51 fighter plane
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Old 10-14-2009, 04:43 PM
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So are you guys suggesting I stay clear
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Old 10-14-2009, 04:53 PM
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I don't know about you guys, but I just love the sound of a 671! Classic Detroit Iron!
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Old 10-14-2009, 04:57 PM
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one of the best engines ever made.
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Old 10-14-2009, 05:15 PM
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Everything is a trade off. I'm like my engines (671TIB'S @485HP). I've had 4 years with only normal maintanence. My engines don't leak as much oil as some people say and I have never had to clean soot off the transom. However I do maintain them on a regular schedule (oil, filters, zincs, cleaning heat exchanger,etc). I installed block heaters. I don't baby them but I never abuse them either. At cruise (2000 rpms for me) I burn approx 22 gals per hour and get about 22 knots with full fuel and crew.

I'm happy with mine. The best advice I can give you is to find a really good DD mechanic and do a complete engine survey. It's money well spent. If you like the boat, get a through survey and factor any work needed into your offer.

John
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Old 10-14-2009, 05:21 PM
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There are a couple of small car ferries on the Sacramento Delta with 6-71 N on a Z drive. They run constant 1200 RPM (I believe) for 2 years, without shut down. (oil refiners and regular oil top off) Then they are majored. They claim to never have a failure--2 years night and day is somewhere about 17,500 hours. Much different service than on a fishing boat.
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Old 10-14-2009, 05:22 PM
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Quote:
Originally Posted by Johnboy View Post
Everything is a trade off. I'm like my engines (671TIB'S @485HP). I've had 4 years with only normal maintanence. My engines don't leak as much oil as some people say and I have never had to clean soot off the transom. However I do maintain them on a regular schedule (oil, filters, zincs, cleaning heat exchanger,etc). I installed block heaters. I don't baby them but I never abuse them either. At cruise (2000 rpms for me) I burn approx 22 gals per hour and get about 22 knots with full fuel and crew.



I'm happy with mine. The best advice I can give you is to find a really good DD mechanic and do a complete engine survey. It's money well spent. If you like the boat, get a through survey and factor any work needed into your offer.

John
Thanks for the advice
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Old 10-14-2009, 06:05 PM
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Quote:
Originally Posted by Oh Reely! View Post
How long should it take for the blk smoke to go away. Wouldnt be cheaper just to do an overhaul on the engines
Make sure you do a cold start/ no block heaters and cool to touch. If they white smoke for more then 30 secs to a minute they are probably getting tired (low compression). Air box inspections are relatively easy to do on the 6-71's. As stated above have a good Detroit guy survey them.
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Old 10-14-2009, 06:51 PM
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Get an engine survey. Hire a competent DD mechanic, do compression checks, look at air box leakage, etc. That is the only way to assess the engine's condition.
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Old 10-14-2009, 07:02 PM
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Quote:
Originally Posted by Oh Reely! View Post
So are you guys suggesting I stay clear
naw, just do your research, www.boatdiesel.com, several DD vets over there. At 4000 hrs, it's probably due an overhaul, but parts aren't expensive(relatively), and readily available. A problem with exhaust manifolds sticks in my head, but may be a different model.

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one of the best engines ever made.
Yep. (sorry). What he said; rock solid, well-known, millions of them out there.

good luck!

Randy
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Old 10-14-2009, 07:05 PM
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Quote:
Originally Posted by Oh Reely! View Post
How long should it take for the blk smoke to go away. Wouldnt be cheaper just to do an overhaul on the engines
Black smoke is typically a load issue: too much prop, too much fuel, too little air, etc.

Randy
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