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Go Back   SailNet Community > On Board > Gear & Maintenance > Engines > Gas > Atomic 4
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Old 10-16-2006
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Exhaust System Repair

I have seen a number of emails floating around about repairing A4 exhaust systems. Mine is both freshwater and seawater cooled. I just went through this repair saga. 'Recognized I had a problem only because my bilge pump was running more than ever before....leaking seawater. I have a '74 C&C in very good condition. An exhaust pipe had rusted through just below the 90 degree elbow where it comes down from the highest point (which prevents seawater back flow into the manifold.) I understand this is the MOST COMMON POINT where an A4 exhaust system will habitually rust through--part "I" of the diagram. Since my boat is very reliable, I decided to PERMANENTLY replace the entire exhaust system from the manifold on back with threaded brass components...rather than just band-aiding it with galvanized parts that might last 5-7 years. Yes, it cost a couple hundred beers for the parts, but my spare time is at a premium for recurrent maintenance, so I am now assured I will actually enjoy future spare time sailing, not replacing this again.
First, I obtained a new manifold flange from Don Moyer which is threaded for an 1 1/4" pipe. Second, The threaded brass components were obtained from www.plumbingfittingsdirect.com who had the best selection of odds and ends and the cheapest price after shipping costs were taken into consideration. Third, I have attached a diagram of the brass components with a few configuration notes. Fourth, I removed and repainted the exhaust/intake manifold unit with high heat engine enamel while I was at it to spruce it up and preclude manifold corrosion. Fifth, you will notice I encased the brass pipes with a lot of high heat coating materials --high heat coating spray, heat shield matting, and insulating wrap--to minimize heat transference. This keeps the cockpit floor cool when the engine is running for extended periods of time.
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Old 11-22-2006
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Two questions:

1) Did you use any thread sealer at the joints?
2) Please describe the swing check valve.

John Feingold
Tartan 30
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Old 11-22-2006
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"Mine is both freshwater and seawater cooled." Presumably you mean, the engine is fresh water cooled with a heat exchanger which is turn is raw water cooled, but the exhaust is ONLY raw water cooled. (I find "fresh" and "raw" to be confusing, and prefer to say "closed" cooling when there's a heat exchanger onboard.)

Brass pipes? For use with seawater and the bonded galvanic connection to the engine block? I'm not sure that's a good idea. I know once-upon-a-time heavy wall copper pipe (damned heavy wall) was used in some raw water exhausts but hadn't heard of brass being used at all. Have you checked into whether that's going to be a galvanic problem to come?

BTW, NICE drawing. Someone must have access to CAD software.
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Old 11-29-2006
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I used Permatex High Temp Form-A-Gasket Sealant at the threaded joints. It is resistant chemical deterioration, will withstand temperatures up to about 600 degrees and is designed for gaskets, flanges and connections. So far, not a single drip.

The swing check valve simply prevents back flow of exhaust fumes and sea water to any portion of the exhaust system in front of the valve. I suspect most plumbing supply houses sell these. It is basically a cylinder with a hinged flap inside that swings in only one direction. Slight water or exhaust pressure applied against the flap opens it mechanically. Water and exhaust flow out the exhaust pipe with no risk of back flow when the engine is not running.

As for galvanic concerns, I have a number of sacrificial anodes installed on the boat...on the propeller shaft, water pump, heat exchanger, etc. So far, I experienced minimal deterioration. The preexisting swing check valve was made of brass and showed virtually no prior signs of corrosion. Brass has a significantly lower anodic index value than does aluminum or zinc, meaning the risk still exists for my brass exhaust system but it won't go before the anodes. If corrosion ever becomes an issue, I will install a galvanic isolator to save my alumimum anodes and other metal parts that come in contact with water.
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Old 11-29-2006
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Skhorleb... Time will tell if I am wrong...but I believe the swing check valve will freeze up and give you problems over the long term in that application. Keep an eye on it.
Do you really only have a 7" rise in your exhust system over the manifold??
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Old 11-29-2006
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Before anyone starts thinking that skhorleb knows what he is doing read the following article.

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Old 11-29-2006
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I infer from your description (that your anodes are aluminum) that your boat is in fresh water?
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Old 12-06-2006
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I live along Mobile Bay in Alabama, so there is no concern with freezing pipes this far south. (It is remarkable in this region that residential irrigation systems are routinely buried just below the turf surface.) When the engine is shut down, most of the water has already been blown out the tailpipe, leaving very little water reservoir to freeze near the check value. Besides, my check value is not quite at the lowest point between brass exhaust and the tailpipe, the tailpipe exits the hull a few inches above the water surface, and my slip is in a very well protected area with virtually no swells that would force much seawater back into the tailpipe.

Nevertheless, it is feasible that some hardcore soul sailing in subfreezing temperatures could encounter a frozen check value, causing the exhaust fumes and seawater to backup into the engine manifold.

The riser actually extends about 8 inches above the manifold port when you consider the tee and the upper elbow. In this instance there is minimal space to extend the riser much higher before the exhaust system would be too close to the cockpit floor for my tastes. To the best of my knowledge this meets the original riser specs.

As for the material found at yaughtsurvey.com, there is always more than one way to skin a cat. Personally, titanium is beyond what I am willing to spend, cast iron has already proven itself to be of inadequate corrosion resistance, while brass yielded a reasonable price point and expected to be of low maintenance. My wife would insist on that 4 karat diamond before I upgrade to a titanium exhaust system.

My boat is sort of in fresh water…in a slip fed by a fresh water “creek”…gets more saline contact once I get into Mobile Bay and The Gulf. I have more trouble with slime on the bottom. The underwater parts remain virtually free of any barnacles. I have not converted all anodes from zinc to aluminum alloy yet but am heading in this direction only because of the low saline environment and the increased galvanic resistance of aluminum compared to zinc. (Aluminum alloy anodes are more corrosive than the brass exhaust components.) I consider this conversion an experiment until I am sure a corrosive “skin” that eventually covers aluminum does not negate its sacrificial intent like painting over anodes.
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