DD 6-71 overhaul and isues

[brico]

I am looking at buying an 1980 Hatteras 37 Convertible twin DD 6-71Ns with 'mere' 3000 hours on them.
Reading through this forum i get a mixed messages on the longevity and the frequency of major overhaul of these engines. While I will do proper pre-purchase mechanical inspection and the boat seem to be very well maintained, I am also interested in what sort of running hours do people typically get out of these engines before having to commit to major ovearhaul?

Reading about these engines and their background and history, they seem to be considered as mules of the north american diesel engine industry and as such I would expect them to last at least 10K hours of modest boating (not blasting the hell out of them throughout those 10K hours) use.

Does infrequent use of boat engines have a negative effect on these engines? It is a stark reality that sometimes we have up to 6-7 months between actual use/load on the engines. (I do not count the late fall, winter and early spring warmups as running the engine on load).

What can be done to prolong the life or reduce the neagtive effect of engines's inactivity except actually running it? Do cold starts affect it greatly? I know that taxi cars can run 4-500K miles before major breakdown, but these are continuously used/running.

 

[rwappleton]

Good Morning,

I can relate my experiences with the 6-71 engines. In my early 20's I spent a short time as a truck driver. My truck was 6-71N powered. I ran the hell out of it with no problems and the company used it for over 20 years with one overhaul. The frame and body wore out the motor was running well when sold.

My boat has 6-71TI's that was a dock queen for 8 years and was only run two or three times a year. I changed all the filters (fuel,oil) oil change, run the rack,checked the injectors (replaced 2) and she runs like a top!

We did have a good engine survey done and the report was they needed TLC but good basic motors.

My opinion is they will provide many years of reliable service IF they survey well. Find a good DD guy and enjoy!

Russ

 

[RT46]

Agree

 

[saltshaker]

If they are in good shape now and you maintain them well, they will out last you. It is much better to run them often but being a NE boater I understand that a layup period is unavoidable. Care for them and don't be afraid to run them. If all is right they can run WOT all day long. Commercial guys who run them will tell you they either run them at idle or WOT.

 

[dsharp]

The reason there are different opinions about 6-71's is there were engines converted for marine use by Johnson and Towers and Covington that were rated north of 450 hp. I believe a 6-71n or natural aspirated were around 200 hp at 2100 rpm. I think some of the turbo charged/after-cooled were turned up to 2500 rpm. In the late 70's to early 80's the 6-71 was about the only game in town except for the 3208 Cats which had similar problems at higher hp ratings. You should get 5000 to 7500 hours out of a 6-71n.

 

[Bill Herrin]

Back in the 70's I had a number of customers who fished Albacore (on the left coast) and got over 10,000 hours from 6-71n's by running them at 1600. Some of the bigger boats ran those engines on generators at 1200 rpm and got twice as many hours plus. The secret (I was told) is not pushing them on rpm or horsepower.

 

[SKYCHENEY]

Our 43DC had 671n's marinized by J&T and they were rated at 310hp. I think that was the maximum output for the N's.

The 671N is thought by many to be the best engine that Detroit Diesel ever built. As stated earlier, I would not be afraid of them if they survey well.

 

[spartonboat1]

I have the J&T 6-71N's in a 43DC 1972. Right engine majored, about 600 hours ago. Will start on a cool day (50 degs) if rolled over a few times, with the stop button held down, until the oil pressure gauge starts to come up. She then usually starts first try, when the stop button is released. On a 65-70deg day starts quickly and settles down fast.

The rule of thumb on this site, aka the poor man's real quick survey, is that the tight engines will start quickly on 65-70deg days, stop visibly smoking, after 12-20 secs when started ( and that is what this engine does). The oil pressure comes up past 50#'s, then drops to 50, at 1,100 idling to warm up. 50#'s is where the oil pressure poppet valve lifts off. If both engines on the boat you are looking at meet these minimum tests, then that is a good sign.

The left engine is original at 2,200 hours. She has three weak cyls, and will start on a cold day of 40-45 degs, but takes a good 2-3 mins of trying. Once started she takes a good 1-1.5 mins to reduce smoking, but after that it is not excessive, but a sharp eye can see a couple cyls smoking a little. If block heated, smoking stops pretty quickly. Therefore I block heat for about 1.5-2.5 hrs, when I can, because she starts quickly and smokes very little. Oil pressure is as good as the right engine, which was re-built, and actually reads a little high at 60#'s at cruise.

I run at 2,150-2,200 rpm cruise, 1,200 hull speed, as I use the rule of thumb of run up to full speed, observe max rpm, then drop back 200rpm. My max is about 2,450rpm, although they are rated for 2,500rpm.

I tech I trust, retired USCG said get all the fuel line and lube lines replaced, as they can go bad internally, which I did at $3,000 or so. The right engine overheated above 2,150rpm for years. Then after 10 years I notice that a main raw water hose had a kink at the pump, due to the pump direction, as installed. So I installed an 'ell' to remove the restriction and voila, overheat is gone and I can run at 2,200-2,300 no prob.

A new set of water cooling hoses is also a fairly inexpensive replacement, which you can do yourself. Upgrade to the "spring" type clamps, if desired; they never become loose. New hoses give peace of mind.

These 2-cycles engines require good pressure in the 'air box', as that pressure is from the the Roots Blowers on their sides, forcing exchange of the exhaust and stuffing a fresh air charge on each stroke. So quick fix, quasi-tuneup, is to have the Air Box cover gaskets inspected. I had many leaking (some almost gone) and it is a fast, relatively cheap fix to replace the gaskets. If the air box port gaskets leak, then the air box pressure is down and they will smoke more and not run as well or make as much power. I notice my original engine does not smoke as much, when warming up, since the gaskets were replaced. I am sure it helped the re-built unit as well. A couple are hard to reach, and I will bet had never been changed, including at the re-build.

The original left engine runs at 175degs, the right (rebuilt) at between 175-180 indicated. I have infrared gunned both engines and they about 175degs.

 

[dastahl]

The 6-71 310 hp versions are very loud. Don't wait for the sea trial to see how loud they are while cruising. Good mufflers and long exhaust pipes help.
My LRC has continuous duty rated 6-71s 185 hp 1800 rpm. With around 15,000 hours no smoke at start up.
skooch