OD34 M25 engine exhaust

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Harlan

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Jun 4, 2004
99
Oday 34 Niantic
Haven't unwrapped the fiberglass insulation tape to verify but it appears that a nipple in the exhaust system has broken. On my boat the exhaust from the manifold flange to the wet hose connecting it to the muffler is 1 1/4" pipe which, while the engine has performed just fine, seems small. My question is replacing it with Sch 80 pipe vs Sch 40. Researching the archives indicates that Sch 80 is OK in general, maybe even preferred, but it is not quite clear to me what any experience is on the M25. I'm quite sure I'm going to find that the failure is at a thread right where it enters the elbow. If my research is correct, Sch 80 would be almost double the wall thickness at the threads. I'm considering replacing the pipe only (3 pieces - maybe 20" total length) and staying with Sch 40 elbows. My concern is the reduction of about 14% less open cross section in the pipe and the added back pressure on the engine.

Anyone with M25 experience or expertise on this?
 
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dlochner

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Jan 11, 2014
13,078
Sabre 362 113 Fair Haven, NY
Can't answer your specific question. However, given the age of your boat, the fiberglass wrap is more likely asbestos. Before removing it wet it thoroughly to reduce the dust and wear an N95 mask.
 
weekendrken

weekendrken

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May 7, 2011
281
C - 30 # 3573 Lake NormanNC formerly Bflo NY
H,

The wrap on our C-30 M-25s is not asbestos, its fiberglass.
Sch 80 is preferred -- the added back-pressure is non-issue.
Galvanized pipe will emit toxic fumes. Use black iron or stainless pipe.
I don't know the type or location of your muffler on the O'D -- I've chartered but never looked at the muffler. But on our C-30s on replacements, we typically use a stainless one-piece riser like below on the M-25 and XP. etc. There's a myriad of shapes and types depending on the model and year.

Also, Universal has a water-cooled riser that is a huge improvement -- no wrap needed and the engine space is much cooler. I have helped some install that riser on our 30s and on a 34.
 
Ralph Johnstone

Ralph Johnstone

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Jan 4, 2006
7,306
Hunter 310 West Vancouver, B.C.
Sch. 80 is will definitely last longer but I'd be worried about hanging all that extra weight off the engine when the engine is turning over at low rev's and trying to re-mount itself somewhere else with the vibration.

Don't know if you're near any industrial suppliers, but I would suggest using all Sch. 40, 316, SS threaded nipples and fittings. This will do a better job of fitting in a tight space if required. If not near an industrial supplier, GOOGLE is your friend (or worst eavesdropper) as the case may dictate. Be sure to use a high temp. thread sealant for this job to avoid CO leakage.
 
H

Harlan

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Jun 4, 2004
99
Oday 34 Niantic
Thanks for the comments. Repaired yesterday. I had done this before some years ago. The system is manifold flange, 2" nipple, then elbow then additional piping similar to the other photos. The 2" nipple was LOOSE where it attached to the flange (not broken). It was installed by threading the nipple into the flange finger tight, then the elbow onto the nipple and tightening the elbow with a wrench which tightens both fittings. Intuitively seems that both ends of the nipple would tighten to the same torque. After a couple years the flange end was loose! The elbow end required a 2' pipe wrench with a 2' extension to break it loose! So what happened to the flange end? It cannot unscrew with vibration. (Once this is assembled nothing can unscrew.) Can the pipe stretch due to vibration? Replaced with Sch 80 so hopefully no future problems.
 
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odaydokay

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Jul 5, 2011
754
Oday 28 Madison, CT
Can you show some photos of the new set up? I have done two black pipe complete exhaust replacements with fiberglass wrap over my now 25 years of ownership (Univ M-12) and dread doing it again. Is there a union in your setup? Was not in mine and I wondered why.
 
weekendrken

weekendrken

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May 7, 2011
281
C - 30 # 3573 Lake NormanNC formerly Bflo NY
Think about the W'beke water-cooled option if it would work with the O'D muffler arrangement.
The M-25 and M-25XP would use the threaded, not v-clamp version of the wetted riseruniv v-clamp riser.jpg.
 
Sefuller

Sefuller

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May 6, 2010
472
1984 Oday 39 79 Milwaukee
Here are a couple of photos of our old exhaust and the new one made with black iron pipe purchased from McMaster Carr. This is from our 39 but I did the same thing on our 34. I purchased some titanium exhaust heat wrap on Ebay to insulate it.
 

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odaydokay

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Jul 5, 2011
754
Oday 28 Madison, CT
That looks roughly like my set up, albeit with a 3 bolt flange. Was more curious about the PVC route. Was also thinking stainless, but black pipe with salt water environment is not a one and done which is what would interest me the most. Looks like no union on your set up.
 
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dlochner

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Jan 11, 2014
13,078
Sabre 362 113 Fair Haven, NY
odaydokay said:
That looks roughly like my set up, albeit with a 3 bolt flange. Was more curious about the PVC route. Was also thinking stainless, but black pipe with salt water environment is not a one and done which is what would interest me the most. Looks like no union on your set up.
Click to expand
The temperatures are too hot for PVC. It will soften and bend. Black iron works and will last a while.

My last boat, a '81 Sabre 30 with a Volvo had a shop built exhaust riser similar to @Sefuller. It lasted about 30 years in freshwater. When it let loose, we were about 14 miles offshore becalmed between thunderstorms in a shipping lane. I cobbled a replacement out of parts from the big box hardware stores in Kingston Ont. and then assembled a new one with 1 ¼" black iron from a big box store and the 180° elbow from McMaster. The 180° elbow has fewer joints and was consistent with Sabre's original riser.

DSC_0060-3.jpeg
 
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