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6-71N Hours?

matt-dyc

New member
Hi all,

Making the switch from power to sail, so new to big marine engines. I have a line on a boat that is otherwise a good value, but I am nervous about the number of hours on the 1980 671s. Port has 3000 hours, no rebuild. Starboard has 1200 since rebuild.

Obviously, if i were to go forward, I'm going to have a survey done and a diesel mechanic go over them, but am I crazy to be even entertaining this? A complete rebuild or repower would not be in my near financial future.

Previous owner was a hard-core cruiser, and live aboard, so I believe him when he says they are well-maintained. That being said, I'm still a skeptic at heart and I'd hate to pay a mechanic to just tell me that it was a dumb idea in the first place.

Any thoughts? Thanks in advance.
-- matt
 
First off...twin 71s in a sailboat??? Must be one BIG boat! I bet it don't have a trailer! Or, did you mean from sail to power?

In my opinion, you will get as many answers to your question as responses. Meaning? Everyone you ask will have a different answer.

A widely accepted "formula" or rule of thumb for marine diesel engine life expectancy is around 5,000 hours with good maintenance. What is "good" maintenance you ask? Good question! And, another one that will garner wide and differing opinions.

With the vastly improved modern lube oils, drain intervals have been going up and up in recent years with no appreciative differences in engine wear or longevity. I've seen off road recommendations for oil changes go from 25 to 50 and to as much as 100 hours between oil changes for some engines. Would you want to wait 100 hours to change your oil? I wouldn't but, then, that's just me.

Here's some perspective from something most people are more familiar with:
On highway miles driven.

I'm an old big rig mechanic and many, many years ago, it became fashionable for truck engine builders to tout 1,000,000 (one million) miles between engine overhauls as achievable. Does it happen? You bet it does. Is it a realistic expectation for most truckers? Absolutely not! A whole lot of things can happen in the life of a truck in the course of 1,000,000 miles. Most of them don't go that far before running into a few things!

But let's look at your question using an arbitrary, but not totally unreasonable, formula using miles driven related to operational hours.

If we were to assume a constant highway speed of 60 miles per HOUR we would then be able to equate your "freshest" engine of 1200 hours as having traveled 72,000 highway miles and your "elder" engine of 3000 hours of performing for 180,000 highway miles. Neither represents enough use to even think about retirement to a truck owner. 500,000 miles is a common point where concern usually starts to set in among most of us that are in the business.

But, all of this is utter nonsense if the engines weren't maintained. That is the key in my opinion. No formula for usage will ever be worth anything if you can't rely on the maintenance data. So, you had better be right when you say that you believe him when he says they are well maintained.

And I always remember what my ol' pappy used to say; "if'n it wears skirts, has tires, floats or flies....RENT IT!
 
Thanks for the response.

First off...twin 71s in a sailboat??? Must be one BIG boat! I bet it don't have a trailer! Or, did you mean from sail to power?

Ha! I'd like to see that sailboat too.

In my opinion, you will get as many answers to your question as responses. Meaning? Everyone you ask will have a different answer.

A widely accepted "formula" or rule of thumb for marine diesel engine life expectancy is around 5,000 hours with good maintenance. What is "good" maintenance you ask? Good question! And, another one that will garner wide and differing opinions.

With the vastly improved modern lube oils, drain intervals have been going up and up in recent years with no appreciative differences in engine wear or longevity. I've seen off road recommendations for oil changes go from 25 to 50 and to as much as 100 hours between oil changes for some engines. Would you want to wait 100 hours to change your oil? I wouldn't but, then, that's just me.

Here's some perspective from something most people are more familiar with:
On highway miles driven.

I'm an old big rig mechanic and many, many years ago, it became fashionable for truck engine builders to tout 1,000,000 (one million) miles between engine overhauls as achievable. Does it happen? You bet it does. Is it a realistic expectation for most truckers? Absolutely not! A whole lot of things can happen in the life of a truck in the course of 1,000,000 miles. Most of them don't go that far before running into a few things!

But let's look at your question using an arbitrary, but not totally unreasonable, formula using miles driven related to operational hours.

If we were to assume a constant highway speed of 60 miles per HOUR we would then be able to equate your "freshest" engine of 1200 hours as having traveled 72,000 highway miles and your "elder" engine of 3000 hours of performing for 180,000 highway miles. Neither represents enough use to even think about retirement to a truck owner. 500,000 miles is a common point where concern usually starts to set in among most of us that are in the business.

But, all of this is utter nonsense if the engines weren't maintained. That is the key in my opinion. No formula for usage will ever be worth anything if you can't rely on the maintenance data. So, you had better be right when you say that you believe him when he says they are well maintained.

So, if maintenance is the key question, will a good diesel mechanic be able to tell the difference? I do know that the PO did the majority of his own maintenance, so I'm not expecting a clear paper-trail.

And I always remember what my ol' pappy used to say; "if'n it wears skirts, has tires, floats or flies....RENT IT!

I have no earthly idea what that means. :)
 
With no record keeping like a log book or materials receipts, even a good archeologist isn't going to be able to decipher the TRUE maintenance history on those engines. A good mechanic will probably be able to give you a good "gut" report by looking at how things are in general in the engine room. He can do things like cut open oil filters and fuel filters to get a feel for how things are NOW and that COULD be an indicator of how things have been done in the past. Detroits are famous for being a bit "messy" with oil leaks and are actually designed to discharge oil from the blower air box and should be allowed to do so. Your mechanic will want to make sure the air box drain tubes are clear and air is flowing from them while the engines are running. If accessible, these engines do have inspection covers on the side of the block that can be removed and you can visually inspect the piston ring's condition. You can't always see everything that is going on because it is just a "window" into the cylinder. A broken ring will sometimes go undetected but you can definitely see how the rings are wearing through there.

You can have a compression test done but be warned that, on most large displacement diesel engines, this is an expensive proposition. Each injector has to be removed, one at a time, and the engine operated on the remaining cylinders while compression is taken in that cylinder. So, it is very labor intensive and takes a GOOD and experienced tech a long time to perform in order to get accurate results.

One alternative for well equipped servicemen is to use an oscilloscope and do what is called a "relative" compression test. The starter current draw is monitored for ramping up and down while cranking the engine. The tool is programmed to assign firing order for the engine and the "current ramps" or spikes associated with each cylinder are then compared to one another. Not an exact test by any means but can be used to isolate a failing cylinder rather quickly.

One thing you could do to protect yourself is to remove an oil sample and send it for analysis. But, that still only gives you what is going on NOW inside that engine. For example, if the oil was just changed, it will probably test as new oil with no contaminants unless you have a holed cooler or a bearing is rapidly failing.

Another test would be for a closed cooling system getting combustion gasses from a leaking head gasket and for the system's ability to hold pressure. Can't be done on keel cooled boats though.

Here's a tale to ponder:
I once sold a 1962 Chevy Impala to a friend. I was really proud of that car and the way it ran. I took really great care of the engine and transmission. I gave my friend a "smokin" deal on the car and I was ABSOLUTELY confident that he would be a happy new Chevy owner. It was a GREAT car!

He called me a week later to ask me how I could have done what I did to a friend. I was confused. "What happened?" I asked. He said that while on a trip to the big city, he stopped and filled up with gas. He got about 5 miles up the road and THE GAS TANK FELL OUT OF THE CAR WHILE GOING 70 MILES AN HOUR!

What??!! I was flabbergasted! How? Well, the car was old, we lived in Illinois and the roads are salted. The tank straps rusted through and I had never really looked at them before.

There was no fire. No one got injured. I offered to fix the car for him. "That won't be necessary" he said. "Why not" I asked. "I junked the car" he said. "Oh no man! I could have fixed that for you". "Well" he explained, "when we flagged a ride into town to try and find a way to get the car fixed, someone came along and pretty much removed as much on the engine and radiator and stuff under the hood that they could get and the car couldn't be repaired."

Moral of the tale? I didn't mean to screw him selling him a used car. Neither of us looked the car over carefully enough to detect the rotten gas tank straps. He declined my offer to give him his money back realizing that it really wasn't my fault that the car was stripped on the side of the highway. But, he was always a bit distant after that. He learned the meaning of the Latin phrase "caveat emptor"...let the buyer beware. I learned not to sell used cars to my family or friends.

As far as my ol' pappy's saying, I don't know what it means either. He just said it. He never explained it:)
 
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