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Old 10-08-2017, 10:18 AM   #1
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Detroit diesel 6-71?

Looking at older OA pilothouses with 6-71s. One has 5500 hrs but 2100 since overhaul. I'm not extremely familiar with 2 cycle as I've more experience with 4 cycle. Pros/cons? Anything to be concerned about ?

TIA
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Old 10-08-2017, 10:36 AM   #2
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Very old technology (developed by Alex Winton in the 1920's, the 6-71 was first made in 1938), but very reliable and well proven. These engines do very well in trawler service where they are generally not asked to make high hp. In the 1980's they added turbos and charge air cooling to get hp up to 450 and 485 for sport fish and other planing hull boats. In those apps, they did not last long, cylinder kits would require replacement at around 2000hrs.

Post what hp rating these are and whether they have turbos. Also post what the owner cruised at regarding rpm, boat speed and gph. That will help determine if the engines are being run in a "sweet spot". If run at say 1400rpm and under 5gph each, they can run dang near forever, 10k hrs and running perfect is not unusual.

They are a little different than four strokes. Exhaust noise can be louder if mufflers are not carefully designed. They sound a little different, some like it, some don't.

They are not quite as fuel efficient as the modern four strokes, but at low rated hp and modest power settings they are very close. Most around 18hp/gph compared to modern four stroke at 19-20hp/gph. The 450 and 485 are a good bit worse at 16-17, but I doubt those are what you are looking at.

But a Jimmy in good shape can be one of the most reliable powerplants on the planet. While out of production now for like 20yrs, there is no problem at all getting the basic engine parts. Some marine package specific parts may be a challenge, but since there are thousands still in service, there is always a way to get what you need.

Prior to buying, you want someone well familiar with these to check them out. The weak spots are well known by these techs, and the symptoms are not the same as on a four stroke. A good DD guy can tell you basic engine health in a half hour. Then a sea trial gets you the whole story.

An absolute classic gem of an engine.
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Old 10-08-2017, 11:20 AM   #3
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Iíve got two in my boat, the are cheep to run, 40 weight and oil filters every now and then. Keep a lot of extra oil around.
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Old 10-08-2017, 11:40 AM   #4
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I was looking at a home built 40 ' Seaton designed steel trawler a year ago. The build was nice but the interior layout was not so good . It had a single Detroit 6-71 in it . Man I wanted that boat.I was on it like a pot a neckbones. I saw the boat a couple weeks ago and it's going down hill quick . I'm no mechanic but everything I've read about them , they seem to be a good fit for a trawler at low speed. Good luck with your search .
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Old 10-08-2017, 11:43 AM   #5
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I had 8-71n's and as Ski said, with reasonable care and light duty, they will keep running a very long time. Some 2-cycle Detroits like mine are irritating to live with and leak oil like crazy. The mechanical noise from the injectors was also very loud. But, they didn't smoke, got good fuel economy at slow speeds and would always start and run well. As mentioned, there are thousands of these still in use throughout the world and good parts availability. I personally l Iike the exhaust note - sounds something like a turbine.
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Old 10-08-2017, 11:44 AM   #6
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Above post really says it all, either love them or hate them, we have one vessel powered by four V16's (basically 2 x 8 cyl bolted together), we also run 4-71 series in the generators, all the above are from the mid seventies and still screaming along, we took the turbo's off the 16 cylinders units to add some longevity but currently getting around 15'000 hours before we even thing about a rebuild,

We just did a full rebuild on a 6-71 (liners/pistons/bearings/seals/rings etc) the parts came in a full rebuild kit for around $1300, so you cant beat that any where, they are sort of known also as being some what leaky (oil) but if kept up with maintenance they are great and damn dependable, there is one issue that we suffered and that is the harmonic balancer (old type) are viscous fluid filled and two of our engines had sat for a time (year or so) and the fluid hardened into a solid causing crankshaft breakages (x 2) before we sorted the issue (not easy to find),

If not run hard enough they tend to "slober" and you get a carbon build up in the air boxes (easy to clean)(if you know) and this gives you a look into the cylinder bores at the same time, the only other issue is setting up the injector racks which can be a bit of a trick, many of the older mech's around know the tricks with this as it's done by feel(although you can use a dial indicator gauge)these engine's use injectors which are the injection pump as well (so no injection pump to worry about) they are commonly available in many various sizes and with timing changes as well you can up the horse power or lower it as you wish,

Damn good engines (although getting dated now)
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Old 10-08-2017, 12:10 PM   #7
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Quote:
Originally Posted by tozz View Post
Looking at older OA pilothouses with 6-71s. One has 5500 hrs but 2100 since overhaul. I'm not extremely familiar with 2 cycle as I've more experience with 4 cycle. Pros/cons? Anything to be concerned about ?

TIA
Why was the engine rebuilt at 3000 hours or so? There maybe some owner practices to consider. Get a price for a complete rebuild from a reputable nearby DD shop. Nice to know.
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Old 10-08-2017, 12:18 PM   #8
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Originally Posted by Ski in NC View Post

They are a little different than four strokes. Exhaust noise can be louder if mufflers are not carefully designed. They sound a little different, some like it, some don't.
Some tournament billfish guys are convinced that two strokes, or at least the big old detroits, raise fish much better than anything else.
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Old 10-08-2017, 12:42 PM   #9
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Hi Tozz,

Based on 12 years of prior ownership of older ('84-vintage) 671's in a Canoe Cove 53, I believe Ski in NC hit the nail on the head. Wonderful engines for low-speed operation ("trawler-like"), if properly set up and maintained. As Ski says, not knowing the HP rating and current setup in the boat you're looking at, it's impossible for others to provide specifics on pros and cons, or what to look out for. There's lots of "urban legend" surrounding these engines, most of which is simply BS. Look to the professionals (and pay them $$ accordingly) for sound advice.

I suggest a very thorough engine survey by a well-respected local engine surveyor intimately familiar with 671s. I suspect this survey will include inspections in each air box to inspect the rings and cylinder bores, a compression check, and an oil analysis. A competent engine survey will provide the answers you seek.

In general, my experience with the turboed and aftercooled versions of the 671s in my boat (410HP each) was very positive. Running a 53' boat at 1350 RPM resulted in a 10-knot boat speed, with a total fuel burn of >9gph. Never failed to start on the 1st roll, never stopped running in 12 years of service. Kept the oil levels topped up, always changed the oil at recommended service hours, and kept an eagle eye on coolant temperature. Parts and service are available worldwide, at remarkably inexpensive prices.

I wouldn't hesitate to own another boat powered by 671s, assuming all was well within them.

Regards,

Pete
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Old 10-08-2017, 01:17 PM   #10
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Just a little footnote about DD's:

In Charleston SC, Hurricane Hugo sank a 75' shrimp trawler with a 12v71. Boat was swimming on anchor and wind actually blew the boat over because of the rigging. Boat laid on its side near marsh with a few feet out of the water on one side depending on tide. A few months later, owner arranged to have it refloated. Took starter off and did the various unsinking chores: water out of fuel, water out of oil sump, water out of gearbox, airbox covers off, bar over to expel water, etc. Got starter cleaned out and some hot batteries.

The dang thing started up and ran!!

The boat came into Shem Creek under its own power. The owner said he had a hard time because of all the oysters and barnacles on the pilot house windows.

All the old salts on the dock were thinking thoughts Biblical upon the sight....

He cleaned up the boat and put it back into service. Ran I think a whole season and he ended up putting cylinder kits in it. Said they needed changing before it sank, so no big deal.
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Old 10-08-2017, 07:49 PM   #11
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671’s are great.
The naturals wil run for ever
Turbos run less time, but rebuildable in place.
If they are jacked up 485 hp run hard expect 2000 hrs max
If they are 185 hp naturals expect 10000 hr +
One of the best motors ever made
They leak oil, smoke when started, but they always start and run.
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Old 10-08-2017, 09:36 PM   #12
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Originally Posted by Ski in NC View Post

Post what hp rating these are and whether they have turbos. Also post what the owner cruised at regarding rpm, boat speed and gph. That will help determine if the engines are being run in a "sweet spot". If run at say 1400rpm and under 5gph each, they can run dang near forever, 10k hrs and running perfect is not unusual.

Thanks for the great info. These are the 671 TI turbo 485 hp. In an OA 548 listed at 52000lbs displacement. Listing also states Cruising Speed: 14 knots @ 1950 RPM and Maximum Speed: 19 knots

I will ask about the prior owner's data.
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Old 10-08-2017, 11:02 PM   #13
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My favorite marine subject.
I bought my current boat because of the DDs (actually Grey Marine) built in 1947. Went 60 years before overhaul. Older than me by a year. Wish they had new pistons and sleeves for me. I've been running 71 series engines since my very early teens. I've owned a bunch of 71 series. Good, reliable engines. No injector pump. Gear pump (lift pump) supplies about 35psi, and the injector has a plunger that makes the pressure. No bleeding issues. Circulates 35 to 70 gph depending on model. Blower makes about 2 atmospheres and intake air come in thru ports at the bottom of the piston travel. Valves are exhaust only.
A natural with decent maintenance will go an easy 10,000 hours. Mine did over 20,000. Turbo versions usually average 3-5000 depending on the operator. I use to get 7000+, but I don't beat my engines and run clean oil. I ran 671s commercially and knew people that bought WWII surplus engines and were still running the original build in the 80s. I do my own overhauls. Did both engines for $5000. I burn 8.5 gph with twins in an 83', 80 ton boat doing 10 kts. 671 TIs burn as much as 30 gph each at WOT.
Keep the engines below 80% of hp, natural or turbo, and you get much longer life.
My engines are Tier 0 and I'll change them when they take the starter buttons out of my cold, dead hands.
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Old 10-09-2017, 07:20 AM   #14
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The 485 HP versions are not long lived if run as they were intended. 3000 hours as previously found may well be a tell. Are they the J&T version?

Prior owner operational and maintenance habits pretty well dictate the 485s current state of health and remaining life. Peruse the boat diesel archives and ask some questions on that forum for additional insight.
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Old 10-09-2017, 09:05 AM   #15
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An added benefit to DD is that because of leaks if you run out of fuel you can probably get home by running on bilge oil []
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Old 10-09-2017, 10:36 AM   #16
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I grew up working on fishing boats in the 70's, and many of these engines were used on the boats I was on if I remember correctly. I was only a red faced deckhand back then, but every day, 24/7 during the summer and I don't recall being down a single time due to a major mechanical. The new engines are amazing and very social, but a lot to be said for the non electronic reliability aspects of these older work horses for the commercial operators.
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Old 10-10-2017, 12:21 AM   #17
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I was looking at a home built 40 ' Seaton designed steel trawler a year ago. The build was nice but the interior layout was not so good . It had a single Detroit 6-71 in it . Man I wanted that boat.I was on it like a pot a neckbones. I saw the boat a couple weeks ago and it's going down hill quick . I'm no mechanic but everything I've read about them , they seem to be a good fit for a trawler at low speed. Good luck with your search .
thanks! Looking at 52-55' trawler or pilothouses for potential long term cruising and liveaboard. Couple of the candidates have the 6-71 most others have cats (3208, 3196 or 3176). Would love to find Cummins but no luck in the older models on the short list so far.
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Old 10-10-2017, 12:39 AM   #18
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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 wouldnít have any issues with 485hp 671s as long as they passes oil analysis.
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Old 10-10-2017, 11:08 PM   #19
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Quote:
Originally Posted by Ski in NC View Post
Very old technology (developed by Alex Winton in the 1920's, the 6-71 was first made in 1938), but very reliable and well proven. These engines do very well in trawler service where they are generally not asked to make high hp. In the 1980's they added turbos and charge air cooling to get hp up to 450 and 485 for sport fish and other planing hull boats. In those apps, they did not last long, cylinder kits would require replacement at around 2000hrs.

Post what hp rating these are and whether they have turbos. Also post what the owner cruised at regarding rpm, boat speed and gph. That will help determine if the engines are being run in a "sweet spot". If run at say 1400rpm and under 5gph each, they can run dang near forever, 10k hrs and running perfect is not unusual.

They are a little different than four strokes. Exhaust noise can be louder if mufflers are not carefully designed. They sound a little different, some like it, some don't.

They are not quite as fuel efficient as the modern four strokes, but at low rated hp and modest power settings they are very close. Most around 18hp/gph compared to modern four stroke at 19-20hp/gph. The 450 and 485 are a good bit worse at 16-17, but I doubt those are what you are looking at.

But a Jimmy in good shape can be one of the most reliable powerplants on the planet. While out of production now for like 20yrs, there is no problem at all getting the basic engine parts. Some marine package specific parts may be a challenge, but since there are thousands still in service, there is always a way to get what you need.

Prior to buying, you want someone well familiar with these to check them out. The weak spots are well known by these techs, and the symptoms are not the same as on a four stroke. A good DD guy can tell you basic engine health in a half hour. Then a sea trial gets you the whole story.

An absolute classic gem of an engine.


That's a good reply Sir! I was just going to say you can't kill a fuel sucking oil leaking 6-71. Great engines if ...

Your response was way better. I'd guess you'd have praise for my Perkins T6-354's.

I love knowledgeable answers. Well done Sir
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Old 10-11-2017, 10:08 AM   #20
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671ís are great.
The naturals wil run for ever
Turbos run less time, but rebuildable in place.
If they are jacked up 485 hp run hard expect 2000 hrs max
If they are 185 hp naturals expect 10000 hr +
One of the best motors ever made
They leak oil, smoke when started, but they always start and run.


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.
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