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Old 04-02-2019, 01:13 PM   #1
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1985 Detroits 4-71 2300 hours

Hello everyone,

I am looking at a 85 Chris Corinthian 425 with twin Detroit 4-71 at 300HP with 2300 hours

The boat also has brand new bow and stern thrusters

She looks to be in great shape and pending an offer and survey

My main concern are the engines. I assume that they are turbos due to HP.

Can anyone help with the reliability on these engines. How many more hours can I expect before major work.

How are they on fuel. All the usual suspects

I thank you in advance

Thanks

Dale
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Old 04-02-2019, 02:17 PM   #2
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I had Detroit 8v71's. Your question on how much life may be left in them can't be answered without knowing how they were treated and maintained throughout their lives, and even then it's a hip-shot. There's a big difference in engine life between running them moderately, or to the pins. Have the engines had oil analysis? Any maintenance logs or history? And then there's the issue of 'marine age' which how long an engine has been subjected to a hostile, corrosive environment in a boat. In your case, since 1985. Go to boatdiesel.com if you're interested in that topic. Finally and specifically to 2-cycle Detroit's, they are loud both in engine mechanical noise (injectors) and exhaust. And many leak oil.....badly. Mine did, but some don't. Bottom line recommendation would be to reduce what you offer for the boat by the cost to overhaul them. The good news there is that parts are avaiable.
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Old 04-02-2019, 02:39 PM   #3
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The 4-71 at 300 is very similar to 6-71 at 450, which is much more common. Not a bad engine, but as above does not live long if run hard. If run at trawler speed, they can live dang near forever.

These are turbocharged and intercooled.

Fuel burn is a little worse than modern four strokes. A four stroke might make 19hp per each gph burned, these will be in the 16-17 range. Not horribly worse.

Parts are available for these. Fairly easy to assess, either do a airbox inspection or a compression check.

Second that about noise, both in engine room and out the exhaust. Hard to muffle these well and that can cause an annoying drone anywhere near the back of the boat.
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Old 04-02-2019, 03:18 PM   #4
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I had Detroits in a previous boat. I thought they sounded great, very throaty. But I did buy a lot of diapers...
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Old 04-03-2019, 12:41 AM   #5
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I've run Detroits for 50+ years. In the military, commercial vessels, as generators in ships, and in yachts. A turbo 71 series can go from 3000 to 7000 hours between overhauls depending on the operator and the maintenance. When run hard, the turbo creates very high exhaust gas temperatures. The high temps cause excessive wear of the rings and sleeves. And it can burn valves. In the past I got 10,000 hours with turbo 71s by keeping the oil clean and the rpms below 80% of hp. Most people can't resist wot. On the other hand, 71 naturals can go 20,000+. I have 2 natural 671s in my current boat that went well over 20,000 before overhaul.

I love Detroits. It's my first choice for any offshore boat because of reliability. They're all mechanical and require no power, no electronics to run. As long as the engine has fuel it will run. Everything on it was designed to be rebuilt in the field. An engine kit for a 471 turbo is about $1000. A good gas engine mechanic with a workshop manual can overhaul it.
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Old 04-03-2019, 05:10 AM   #6
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4/71 DD @ 300 HP seems to be a little over and above what I would expect 4/71 to put out, does it have a turbo ? normal HP 1/2 that. I run a pair of 12V71N 's DD, got over 22000 hours last rebuild was in 1989 removed turbos at that time engines used to much oil, went back to Natural's . last fall went from southern California to Port Angles used 1 liter of oil in Port engine. Smoke on start up only as for leaking oil not a problem that I have had, run hard/leak hard.
Great engines ! Oil getting harder to find only one source that I am aware of, Chevron, Dello 100, not cheep either, I change oil and filters every 100 hours or so, oil is the life of those engines.
The longer you listen to a pair of Detroit Diesel, the quieter they get !
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Old 04-03-2019, 06:37 AM   #7
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"Great engines ! Oil getting harder to find only one source that I am aware of, Chevron, Dello 100, not cheep either,"

Rotella T is common and has the required CF-II rating in SAE 40 wt.

With non commercial use, the usual 200 hrs a year a 71 N should last 100 years !

One big delight is the air box covers can be removed easily , fogging oil sprayed into each cylinder for out of service time.

Ours sit for 6 months , intake and exhaust sealed ,then smoke like crazy on first new season start as the fogging oil is burnt out..

The only caution to out of storage relight is to pull the valve cover (4 hand knobs) and observe that all the injectors move free with throttle movement .

If operated at under 1800 RPM , with the test ability to see 2100 underway they are very long lived engines.

THe turbo versions if run hard will have shorter lives.
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Old 04-03-2019, 06:39 AM   #8
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4-71 naturals were in the 140-160hp range depending on what reference you use. You've either got naturals and a misprint (150hp each, 300hp total), or you've got really hot turbo 4-71s. For comparison, 8v-71 naturals (twice your engine size) are 320hp each. If you have 4-71 naturals, 10-20,000 hrs if maintained. If turbos (and likely inter and after cooled), 3-5,000 hr life expectancy. Natural Detroits will almost literally run forever. Hot turbos have a very short life expectancy.

FYI, Shell Rotella T1 straight 30 and 40 wt is just under 1% Ash and therefore approved for Detroit Diesels. I have no problems getting Shell through Napa, though they don't usually stock it in the store. If I call in the morning they usually have it sent over from their warehouse to the local store by 3pm same day.

Delo 100 is lower Ash, but I have a really hard time finding it.
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Old 04-03-2019, 08:34 AM   #9
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I appreciate all the sound advice.
I will have an oil analysis done and a mechanical inspection including a compression test prior to purchase. I didnt know that these engines are 2 stroke. That concerns me some
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Old 04-03-2019, 09:27 AM   #10
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Quote:
Originally Posted by Dhauz View Post
I appreciate all the sound advice.
I will have an oil analysis done and a mechanical inspection including a compression test prior to purchase. I didnt know that these engines are 2 stroke. That concerns me some
No need for concern.

The DD 2-stroke engine is quite possibly the most successful engine ever made.
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Old 04-03-2019, 09:45 AM   #11
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Yep, two stroke. But don't be put off by that. A two stroke diesel is not like a two stroke outboard. They are a little different than a 4s diesel, but not enough to make one shy away.

And regarding the 4-71 at 300, DD absolutely did make and market these for at least a few years. Not a lot of them out there, but some. I think some were rated 280 or other hp too. Still a good machine.
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Old 04-03-2019, 02:52 PM   #12
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Originally Posted by Commander Nomad View Post
4-71 naturals were in the 140-160hp range depending on what reference you use. You've either got naturals and a misprint (150hp each, 300hp total), or you've got really hot turbo 4-71s. For comparison, 8v-71 naturals (twice your engine size) are 320hp each. If you have 4-71 naturals, 10-20,000 hrs if maintained. If turbos (and likely inter and after cooled), 3-5,000 hr life expectancy. Natural Detroits will almost literally run forever. Hot turbos have a very short life expectancy.

FYI, Shell Rotella T1 straight 30 and 40 wt is just under 1% Ash and therefore approved for Detroit Diesels. I have no problems getting Shell through Napa, though they don't usually stock it in the store. If I call in the morning they usually have it sent over from their warehouse to the local store by 3pm same day.

Delo 100 is lower Ash, but I have a really hard time finding it.
NAPA sells Mobil 40wt low ash oil in 5gal pails for $84, I just got 7 of them since they were running a 20% AAA discount. Ask for part # NP75118.

BTW my 12v71s at 3 times the displacement only make just over twice the hp (650).
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Old 04-03-2019, 05:22 PM   #13
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Quote:
Originally Posted by Woodland Hills View Post
NAPA sells Mobil 40wt low ash oil in 5gal pails for $84, I just got 7 of them since they were running a 20% AAA discount. Ask for part # NP75118.

BTW my 12v71s at 3 times the displacement only make just over twice the hp (650).
Tried that NAPA part number, came up no good. Can you check and repost it?

Also, is that Mobil oil API CF-11? What's the ash content?

Thanks. I'm having a hard time getting oil as well.
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Old 04-03-2019, 06:28 PM   #14
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Click image for larger version

Name:	EBF51818-14F9-48E2-A1E1-6EB74CF28FA2.jpg
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Size:	111.0 KB
ID:	87184

Like everyone else, it’s sideways.

This is what the guys at the Hatteras site suggested. Try searching 75118 alone. The NP probably just means NAPA, when I call to order it I just ask for 75118 and they have it in 24 hours at the most, usually later that same day.

BTW, this is the best pail pump I’ve been able to find, it’s the only one with a full set of adapters to fit any pail:

https://rover.ebay.com/rover/0/0/0?m...2F382051169314
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Old 04-03-2019, 08:25 PM   #15
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A 300hp 4-71 scares me. However if it spent its whole life only making 100 hp then it’s a real non issue. If there was ever an inexpensive diesel rebuild it would be a 4-71. I think the oil analysis will be the answer to this engine.
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Old 04-03-2019, 08:36 PM   #16
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Oil analysis won't show liner/ring condition on these. Still worth doing to look for coolant/fuel/salt/etc, but you can't read much into the analyses. Ring/liner wear products get carried out with scavenge air and don't go back into the sump oil.

Gots to do a cold start and airbox inspection and/or compression check.

Warm engine will crank and run fine with broken or stuck rings. Sneaky brokers know to warm things up before you get there.

Tell them explicitly that the engines shall be cold on your arrival or you will walk.
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Old 04-04-2019, 06:54 AM   #17
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"I didnt know that these engines are 2 stroke. That concerns me some."


Don't be concerned , there are still valves to adjust!
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Old 04-04-2019, 08:36 AM   #18
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You need to read this: https://www.samsmarine.com/forums/sh...-DD-by-Genesis

Genesis’s 30 second compression test in detail.
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Old 04-04-2019, 08:43 AM   #19
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Attachment 87184

Like everyone else, itís sideways.

This is what the guys at the Hatteras site suggested. Try searching 75118 alone. The NP probably just means NAPA, when I call to order it I just ask for 75118 and they have it in 24 hours at the most, usually later that same day.

BTW, this is the best pail pump Iíve been able to find, itís the only one with a full set of adapters to fit any pail:

https://rover.ebay.com/rover/0/0/0?m...2F382051169314
Thanks, I'll check out the Napa pail next time, after I go through the 4 cases of Rotella I have stocked up.

Regarding pumps, a simple valve manifold connecting the oil pans of each of my 3 engines (2 mains and gen) to this pump makes life clean and easy.

Crack a beverage, flip a switch, and pump out the old to the container of your choice. Change filter, reverse switch, and refill from the container of your choice.

OP-7 Impeller Pump
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Old 04-04-2019, 08:49 AM   #20
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That’s the system I use for my oil changes. The hand pump is for servicing the oil level between changes.
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