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Old 07-15-2016, 10:26 AM   #21
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Being two stokes, a six cyl fires six times per rev. A four stroke like a Lehman fires three times per rev. That's part of the reason for the distinctive exhaust note. Much higher frequency than a four stroke given the same rpm. That and the blower is super loud, and even with a silencer a good bit of noise is from the blower.

And 12-71 fires 12 times per rev!!

Basic design dreamed up by Alex Winton, a Cleveland motor car builder. GM bought him out in the 30's when they started to sense the promise of his designs. Same design used in EMD locomotive engines, many still in service today.

Take a DD apart and it is amazing how complex the block casting is. Not just a chunk of iron with holes drilled in it like a four stroke. Humbling that they could do that like 40yrs after the Diesel was invented. And they are still in service 120yrs later.

It won't be long and someone running a Detroit is going to look at the caledar and realize there engine is 100yrs old!!
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Old 07-15-2016, 10:57 AM   #22
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Sorry for the hijack.
I've just come to the conclusion that I have to replace the return line on one of my 6-71s. It's 5/8" blue hose. Original from Viking. (Circa 1987) Anybody know what it is?
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Old 07-15-2016, 11:01 AM   #23
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This and the "best trawler engine" thread are really good reads.

Ski;
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Originally Posted by Ski in NC;
Humbling that they could do that like 40yrs after the Diesel was invented. And they are still in service 120yrs later.

It won't be long and someone running a Detroit is going to look at the caledar and realize there engine is 100yrs old!!
I came across this tale last night;
The Detroit Diesel - the iconic American high speed two stroke diesel engine

And by the way, I know you would get tired of me but I sure would like to have you as my neighbour. I'm like a an old mop with everything you write.

And Oscar;
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I've heard it described as three Harleys starting up at the same time......
An absolute fallacy. You can't get 3 HDs to start at the same time.

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Old 07-15-2016, 11:12 AM   #24
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an absolute fallacy. You can't get 3 hds to start at the same time.
lol
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Old 07-15-2016, 11:15 AM   #25
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Detroits leak enough that if you run out of fuel you can get home on bilge oil.
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Old 07-15-2016, 12:31 PM   #26
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Detroits leak enough that if you run out of fuel you can get home on bilge oil.
I hear this all the time but reality is mine don't leak at all.
Save and except the calamity caused last year when the yard decided I didn't really need that second crankcase vent and removed it.
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Old 07-16-2016, 12:08 AM   #27
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Sorry for the hijack.
I've just come to the conclusion that I have to replace the return line on one of my 6-71s. It's 5/8" blue hose. Original from Viking. (Circa 1987) Anybody know what it is?
I had the same lines on my boat and they held up wonderfully for years. However, when I had the boat surveyed I was told that I needed to replace them with metal lines (copper?). Perhaps that is the USCG requirement? You may wish to check because this would be a good time to upgrade.
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Old 07-16-2016, 12:11 AM   #28
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I hear this all the time but reality is mine don't leak at all. Save and except the calamity caused last year when the yard decided I didn't really need that second crankcase vent and removed it.
I never understood what people meant by DDs leaking. I placed a small tin can under each crankcase breather and they never filled to the brim. The 6 inspection ports on the block dripped a bit but I wiped them down after each run. No big deal. BTW, the "factory" gaskets under those port covers are horrible. I've always thought that replacing them with something more spongy/compressible would eliminate those leaks 100%.
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Old 07-16-2016, 08:37 AM   #29
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Build a Detroit carefully, and they won't leak. Well, maybe a drip here and there. The leakers are the ones put together sloppily, mostly due to poor cleaning of the mating surfaces and seal areas.

The thing is, if any other engine was built sloppily, it too would leak IF IT RAN, which it probably won't for long. The DD will run!!!
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Old 07-16-2016, 12:50 PM   #30
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The problem as Ski notes is two fold , some skill IS required to work on a DD ,
\
but many factory gaskets stank, and worst of all the sealants DD chose were lousy.

MY DD 6-71 is a factory fresh 1950 era "war reserve" engine taken from the storage can in Y2K or so that has never yet been apart for repairs , so is tight.

Having lost many contracts to the euros DD cleaned up its act ,

but now that they have been taken over by the Germans , good sealants and good gaskets are normal.
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Old 07-17-2016, 07:01 AM   #31
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Love DD's. Have 6-71Ti's in my boat and had 6-71N's on my last tug as generators. Feed them fuel (even dirty) and they will run. FF is right fuel burn depends on injector size. The fuel chart Mako has is the one I have. It does not coincide with my fuel burn. I have had no need to pull an injector so I do not know what size I have but @1100 rpms and 8 kts, with a 15kw generator at about 30% load I burn a TOTAL of 6.13 gph. I round up to 7. At 1800 rpms 14-16 kts, I am burning 18 gallons an hour total. That is a 50k lb sundeck motoryacht, she will hold there all day without missing a beat. Now the naturals on my tug we ran them for 6 weeks straight 24/7, they never missed a beat either. I would always buy a boat with DD's. A buddy and 40 year tug Captain told me don't touch them if they are running well. Change the oil and change the coolant. Oh yea about oil changes, I know most people say every 100 hours but that's bunk, 250 is good, hell we go longer on the tug. I know I'll get flamed for that but can't argue industry knowns.
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Old 07-17-2016, 11:45 AM   #32
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Yep, I think most change the oil too often, not just DD's but other brands as well. I do my Cummins at about 300hr and oil sample indicates that even that is conservative. I only change it at 300 because beyond that I start thinking about it too much.

If your oil temp never gets high enough to cook off moisture, then yep you should change it often. DD's and Cummins and Cats usually keep the oil temp nice and high so moisture stays cooked out.
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