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Old 03-03-2017, 05:03 PM   #1
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cracked wet exhaust manifold, why?

Our boat sits for months at a time and prior to leaving it I run the engines and genny at the dock until each reaches operating temp. When I did this last visit everything seemed fine, the 3208 cats purred like kittens as did the Northern Lights 30kw genny.

This visit I fired the genny up and noted water spraying from the wet exhaust manifold. After shutting down and pulling it off I found it was very severely cracked on the exhaust side from the hose flange all the way up to the water connection. The crack had opened up nearly 1/4 inch and is twisted. ie: the two haves would not line up if you could somehow squeeze them back together. As well, the cooling water side also had a long hairline crack along its length. It looks exactly like one would expect if it had froze but this is impossible as it is in Florida.

Any ideas what may have caused this? It was definitely not leaking when I left, this is a huge crack and would have been very noticeable, what could make it split just sitting there? I have never seen it overheat and regardless of load it generally runs about 105F. Not sure what the temp spec is but I always check for cooling water flow immediately upon starting.

All thoughts welcome, thanks!
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Old 03-03-2017, 05:27 PM   #2
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Is it cast or fabricated? Is the metal thin and rusty, or did it split thru good metal?

If it was a welded fabrication, it could have residual stress from assembly that caused the split once it rusted thru enough for it to give. . .

My 2 cents. Do you have any pictures?
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Old 03-03-2017, 05:43 PM   #3
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Any chance it froze while you were away?
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Old 03-03-2017, 07:25 PM   #4
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Any ideas what may have caused this? It was definitely not leaking when I left, this is a huge crack and would have been very noticeable, what could make it split just sitting there? I have never seen it overheat and regardless of load it generally runs about 105F. Not sure what the temp spec is but I always check for cooling water flow immediately upon starting.

All thoughts welcome, thanks!
I recall a NL issue with exhaust manifolds cracking, here's an excerpt from an exchange with Bob Senter of NL...
1. Cast iron exhaust elbows are still cast iron on most generators. They have a very satisfactory service life on commercial vessels where the machines run with heavy loads and seldom sit unused. They generally donít last as long in recreational applications where sustained light loads cause carbon loading issues (cracking and exhaust restriction). Extended downtimes promote corrosion and internal rust.
2. Cast stainless steel elbows are available for service replacement at the same price as the cast iron elbow. Cast stainless isnít perfect either; occasionally an originally undetectable porosity may develop after some time in service. Fabricated stainless elbows have similar issues at the welds so there is no truly permanent solution. (By the way, NL also buys fabricated SS elbows from DíAngelo which are furnished as OEM on larger engines.) But, the cast SS elbows are have proven to be quite satisfactory and seem to last a long time, while being affordable to produce.
That may shed some light on the failure, particularly if your genset is subject to the aforementioned sustained light loads.

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Old 03-03-2017, 07:31 PM   #5
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Yep, many NL gennie mixers are cast iron. The corrode internally, and eventually it cracks the water jacket part and it sprays. Check with NL. On many models they now have a cast SS mixer that has a much longer service life. Just put one on a few years ago on my 5kW. Cost was reasonable.
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Old 03-04-2017, 09:55 AM   #6
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Thanks for the input guys, I think that little gem that Maerin (steve) posted from NL says it all. (where do you guys find this stuff !) My genny only runs 3 to 4 hours a year and it is so oversized it is impossible to put a good load on it.

Incidentally it is cast. Strangely enough the cracked elbow is fairly clean inside with minimal rust. Oh well.... I needed another boat project anyway. I'll replace it with stainless
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Old 03-04-2017, 10:34 AM   #7
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CK, Per chance you ran our engine without the through hull open?


The NL elbows are like so many other boat parts, wear items. Waiting until failure is not always a good idea especially if genset is in a sound box and not always seen every day.

After replacing SS main engine missing elbows (small drips and originally 304 SS) after 8 years of service I though it best to replace the Westerbeke genset elbow. It is aluminum and looked perfect! It is now an emergency spare.

Ski, you have talked on these Al elbows before. Maybe the metal is less important than the use cycle do you think??
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Old 03-04-2017, 11:03 AM   #8
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CK, Per chance you ran our engine without the through hull open?
My first thought too.

Or having a no flow situation that is rectified. Heat up the exhaust elbow, add cold water and thermal fatigue occurs in the form of cracking.
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Old 03-04-2017, 11:11 AM   #9
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Greetings,
Mr. CK. Can the piece be welded/brazed. I realize this may be considered a band-aid but...
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Old 03-04-2017, 11:21 AM   #10
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Yep, many NL gennie mixers are cast iron. The corrode internally, and eventually it cracks the water jacket part and it sprays. Check with NL. On many models they now have a cast SS mixer that has a much longer service life. Just put one on a few years ago on my 5kW. Cost was reasonable.
I'm planning on pulling the mixer off my 8 year old Northern Lights as part of spring commissioning this year. I'm definitely planning on the cast stainless replacement if needed. I think its less than $300.

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Old 03-04-2017, 02:04 PM   #11
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The part number that I found for the Northern Lights mixer is 27-32003.

I'd be interested in any casting numbers on the mixer.

32003 happens to correlate with a grade of duplex stainless, which would be a great material. But as I don't know the cast equivalent, this might just be wishful thinking...
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Old 03-04-2017, 03:15 PM   #12
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The part number that I found for the Northern Lights mixer is 27-32003.

I'd be interested in any casting numbers on the mixer.

32003 happens to correlate with a grade of duplex stainless, which would be a great material. But as I don't know the cast equivalent, this might just be wishful thinking...

Same as my M753W2 would take. (it apparently fits a lot of their gens) I looked it up, its actually $353, but seems reasonable for a factory replacement especially in stainless.

27-32003 | Northern Lights Wet Exhaust Elbow | Diesel Specialists, LLC

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Old 03-07-2017, 06:58 AM   #13
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"The crack had opened up nearly 1/4 inch and is twisted. ie: the two haves would not line up if you could somehow squeeze them back together."

I would attempt to figure out where this high stress came from before re assembling.
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Old 03-07-2017, 07:31 AM   #14
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Corrosion products build up inside. The corrosion products have a volume at least a few times the metal from which they arise, causing the remaining material to overstress and crack. There really is not much of a limit to the forces involved. The volume of corrosion products increase due to chemical action, and the space for them to occupy is fixed. Cracks form and open up.

The part at that point is done. No brazing or welding will fix it, at least properly. Replace it.
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