Rebuild vs replace diesel engine - SailNet Community
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post #1 of 41 Old 02-22-2008 Thread Starter
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Rebuild vs replace diesel engine

Latest copy of (Australian) Cruising Helmsmen has an article replacing or rebuilding a diesel engine ("Fix it or flick it"). Not a bad article, however one section I don't agree with is this statement.

The first question to ask is: how old is the old engine? If it's heading towards the 20 year mark then even talking about a rebuild is probably not a good idea.

The article gives a couple of other reasons for not rebuilding (ie damage to the crank shaft, pistons rings, etc) however the main issue stated is age with 20 years being the magic number. I would of thought a number of other issues would come into the decision including:
  • engine hours
  • how well the engine has been maintained
  • how easy it is to obtain spare parts
  • engine access, ie can the engine be lifted out easily or do you have to dismantle half the boat to get at the engine
  • etc, etc

The two most important ones would be engine hours and availability of spare parts.

For example, my boat has a 98HP six cylinder Nissan SD33 diesel engine installed in 1979 (29 years old). Engine is keel cooled (ie cooling water circulates though pipes mounted on the outside of the hull - no saltwater through the engine). Engine has around 2,700 hours, of which I have done about 150 hours over the last 2 years. When I brought it two years ago had it inspected by a diesel mechanic, who gave it a clean bill of health. Have had no problems with the engine during this time. Spare parts for this engine are readily available (it's the same engine that was in the Nissan Patrol from around 1981 to 1989).

Over the next 2-3 years one plan is to remove the engine as part of a major boat rebuild and have it either overhauled or replaced, depending on how the engine has gone, what the inspection shows, etc. However according to the article replacement would be the only option.

Any advice / opinions / comments?

Ian (ilenart)
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post #2 of 41 Old 02-22-2008
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Wondering maybe if such things of world wild availability of parts, the emission issues, fuel economy, weight and size might be in there as well. I understand the old statement if it's not broken why fix it. The replacement might also lead to new spins offs as wiring updates, easier controls to just mention a few.

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post #3 of 41 Old 02-22-2008
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I think..

Not having read the article my guess would be they say 20 years because at that point the following items are pushing their odds on a boat..

1) Alternator
2) Motor Mounts
3) HX if equipped
3) Starter
4) Tranny
5) Injection pump
6) Water pump
7) Fuel lift pump

Basically, around here, a diesel engine rebuild includes the internals of the engine, machining of any internal parts and the gaskets required to put it back together. These quotes usually only include new rings and gaskets. If you happen to need a new cyl head or pistons the price is on top of the quote because they don't know until they get in there. These re-builds usually DO NOT included the injection pump unless you specifically ask or any other part of the motor for that matter.

My neighbor recently, well three years ago, re-built his Yanmar 3GM and the total re-build cost was over 5k. All he got was R&R of the motor, new rings and gaskets and internal machining and a little spray paint. Suffice it to say most re-builds do not re-build the starter, alternator, water pump, acid dip the HX or even look at most if not all external components..

For 5k every other component on his motor was still pushing 20+ years or original. At that time he was quoted 7k for a brand new motor with transmission and all associated parts. Since that happened, he has continually belly ached about how he should have purchased a new motor because his tranny, alternator, motor mounts and a few other items have since let go too. He's spent well over the price differential between new and re-built in the tree years since he had the Yanmar overhauled.

There was a time when a re-build was worth the differential but with labor rates pushing $100.00 US per hour, and parts cost as high as they are, that differential is very easily eroded and not always worth it when you consider the age, and cost, of the external components when purchased individually..

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Last edited by Maine Sail; 02-22-2008 at 07:30 AM.
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post #4 of 41 Old 02-22-2008
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Ian, I don't see any mention but one big question is whether the engine was salt-water raw water cooled. Raw salt water can eat out cooling passages in the engine, making it incredibly expensive or totally impossible to rebuild. After enough years--it just doesn't pay to gamble if that's part of the question.
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post #5 of 41 Old 02-22-2008
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I think I can understand why they would say that. Technology over the past 20 years has greatly changed. If the break point on cost is within budget I'd go with a new engine and gain the rather considerable savings in weight and fuel economy and count the controls and monitoring capabilities as gravy.

You might be able to find plenty of spare parts for that old Ford (for example) but can you change the impeller in 5 minutes like I can on my new Westerbeke that I've fitted with a swap in thumbscrew quick change unit?
Can you plug it into a NMEA 2000 network and monitor it on a laptop for compression, fuel flow, power, temp etc - that really lets you know what's going on inside it - or do you troubleshoot problems by ear and smell?
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post #6 of 41 Old 02-22-2008
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Our Volvo-Penta MD11C two cylinder Saildrive had no compression or power.
30 year-old Volvo parts are hard to come by and we waited nearly 9 months for components to replace the cylinder liners and rings. After all this time we bit the bullet and purchased a modern 4 cylinder saildrive for $13k A hard decision but full time ocean cruising demands a reliable auxillary in the boat.
We'll have that soon, launch day is fast approaching and our Niagara 35Mk1 will have wings with that 39 hp !!!
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post #7 of 41 Old 02-22-2008
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With the cost of a rebuild a very substantial fraction of the cost of a new engine (installed), and assuming you're looking to keep the boat for the foreseeable future, you might consider how you evaluate the two options from the perspective of ten years from now:

Option 1: Spend X for a new engine and ten years from now you have a ten year old engine with about 1000 hours in service (well maintained, that's almost as good as a new engine).

Option 2: Spend 70-80% of X to rebuild the current engine (assuming you have it done professionally, that's probably a reasonable estimate of what you'd spend) and you then have a 39 year old engine with 1000 hours since a rebuild and 3700 hours of total service.

Shift perspective slightly....assume you're trying to sell the boat ten years from now. How would a prospective buyer look at the relative value of these two options?

Finally, there's always the 'do nothing' option -- if it's working fine, keep loving it and replace it down the road when the engine's performance warrants. At 75-100 hours a year you could probably live with what you've got it for a long time. On the other hand, if you're doing the refit in preparation for a long, multi-year voyage to the far corners of the earth, then focusing only on the rebuild vs replace options probably makes sense.

Last edited by billyruffn; 02-22-2008 at 10:47 AM.
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post #8 of 41 Old 02-22-2008
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Another issue from professional mantaince stand point is that the rebuild/ replacement is suposed to last another twenty years. Parts that are avalible now may not be in ten years. Having worked on a 125' shcooner that used older technology so that it would match it's sister ship, I can say that you'd be better off replacing it. extra parts may look avalible, but when they show up will have to be modified or will have a shorter life then intended.

An example would be that we had to replace most of the silenoids on the main engine and one of the generators just about every other year. This was because our's where originally 32 volt and the replacements we could get were 24 volt. They worked but would fry out petty quick. All are electrical componants had to be sent out for rebuild. It was rare to find a new or already remanufactored sitting on a shelf. Any time something broke we'd have to either jury rig a temporary sulution or put the boat out of service untill we had the parts rebuilt or refabricated. Even simple items like the pump braket or the transmission linkage, would have to be fabricated or retro fit.

Thats what you get into. It may be that you can still get impellers but there half again as much, only made by one company, and made using cheap rubber. Not like you can go somewhere else to get them. Same goes for gaskets, random brakets, control linkages, electronics, and just about all the stuff that should be no biggy, but becomes a major hassel.

The other thing to consider is doing all the other refitting you have planned including deciding what new motor you'll use when you repower, then leaving the existing engine in place till something big goes.

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post #9 of 41 Old 02-22-2008 Thread Starter
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Thanks guys for all the input. Much appreciated.

Hellosailor, engine is freshwater cooled via keel pipes. No saltwater through the engine. Saltwater does cool the gearbox via a heat exchange then out through the exhaust, however this system is separate from the engine.

Billyruffin, your "do nothing" option is basically what I am planning on doing for the next two to three years. The replace /rebuild is one option that may come up after that.

Know one has commented on the number of engine hours. My understanding is that engine hours as opposed to years is a better estimate of engine life, which is the main issue I had with the article. I have often read that you would expect 5,000 to 8,000 hours from a diesel engine before you would expect a major overhaul. What I am thinking (hoping?) is that a detailed examination of the engine shows no major problems / issues. In that scenario I believe reinstalling the engine would make sense, as the engine would still have 40-60% life left.

However, if the detailed examination showed major problems then yes a new engine would make sense.

PS, just reading some of the posts in "Theory of fibreglass blisters" phew!

Last edited by Ilenart; 02-22-2008 at 12:42 PM.
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post #10 of 41 Old 02-22-2008
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The reason you can't use engine hours after say year fifteen is that time also takes it's toll. The same in cars. Gaskets deteriate, metal corrodes, and even the insulation on the internal wireing goes bad with time. Your parts including both the rubber and metal ones have now been soaking in a slightly toxic envionment for a long time. Plus the short uses it's most likely to have expeirenced is harder on an engine then long hours are.

It's the same in cars. A 1973 ford truck with only 45,000 miles will have as many or more problems then a 2003 ford truck with 145,000 miles. In the end if you think your engine, transmission, and the other parts that are attched to it are in really good shape then rebuild it. But there is an old adage that goes " You can put a rebuilt transmission on an old engine, no problem, but a rebuilt engine on an old transmission will blow with in two years."

Same goes for rebuilding the top of an engine without rebuilding the bottom. Your basicly going to put a new power plant onto a worn transmission, as well as impellor pump and altenator. This engine's going to turn faster and give more tourqe then the old tranmission can handle. The bearing in the altenator are older and have fatigued both from they're use and from corrossion and the leaching out of carbon and iron that make up the steel.

Thats why its not recomended. Plus for a couple of bucks more you get to advertise a new drivetrain when you go to sell it. A big advantage because most people are afraid of getting stuck with a blown tranny or engine. Add in the fact that the new stuff comes with a longer warrenty on more peices then the short warrenty that you'd get on a rebuild. And you can see why after twenty years no one really goes by the hours any more.

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