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Help me out here. I've seen many threads about replacing exhaust manifolds on a semi-regular basis. I have never changed the manifolds on my Mercruiser 260 in 7 years. At three year intervals I have removed the manifolds, disassembled them and taken them to the local automotive/engine machine/rebuild shop where I have them hot-tanked. They are then reassembled with new gaskets and grade 8 bolts, repainted, and reinstalled, costs $50 bucks
The boat is used mostly in freshwater, maybe 6-7 saltwater trips per year. I do plan my salt water trips so I can return home with enough time to launch at the local lake for a couple of hours before going home or I do it the next day.I consider the$20 launch/park/use/fishing fee cheap to avoid the hassel of a muff flush in my driveway.
When I store the boat for winter I drain the engine and manifolds, leaving all of the drains open until Spring. So far, I've never had a leak, and have never had to replace any of the freezeout plugs either.
I guess the question is "am I asking for trouble by not replacing the manifolds?" I realize the cooling system is 'abrasive' - but I have not found any 'wear' in the manifolds themselves. The thermostat has been replaced and some hoses from time to time, but I've never had a cooling problem with the engine.
What are your never to be humble opinions?
__________________ NJM
If you are not the Lead Dog the scenery never changes.
21' SeaRay Cuddy w/Mercruiser 260 I/O
When I store the boat for winter I drain the engine and manifolds, leaving all of the drains open until Spring. So far, I've never had a leak, and have never had to replace any of the freezeout plugs either.
You're better off filling the cooling system with non-toxic antifreeze. Air and moisture will rust internal passages of the cooling system. Antifreeze will remove the air.
BTW: They are not "freezout plugs", they are there to fill holes made by the casting process. Counting on them to pop out and prevent freeze damage to an engine block is asking for trouble.
As for the manifolds and risers, it's hard to say as they deteriorate from the inside out. Most folks would recommend replacement at seven years. Fresh water will corrode cast iron, just not as quickly as salt water.
When I lived on Lake Michigan manifolds used to last 20 years or better. Down here in SW Florida manifolds will go at least 4 years or better. My last boat had over 1000 hours on the manifolds and they still flowed well and no sign of temperature change.
If you run the boat thru fresh water after salt water, I would not even worry about pulling them at the end of the season.
This is my experience and no other facts to go on.
......... My last boat had over 1000 hours on the manifolds and they still flowed well and no sign of temperature change ....................................
It's not the temperature change that gets you, it's the hydrolock that stops the engine when the cylinders fill with water.
You have water surrounding the exhaust passages. If that wall rusts through, water enters the cylinders. Since water cannot be compressed, something will bend or break. This can be a pretty expensive repair.
Makes sense, then the issue is how comfortable am I with the worst case scenario. For piece of mind replace them. However I do believe the manifolds do have a lot of years left in them, if they are a fresh water run boat.
If theres no heavy rusting\flaking internally there fine. Since your running in fresh water they should last much much longer as thekent mentioned. Freshwater will take forever to rust cast iron. Look at it this way- Look at the 100 year old cast iron radiators in homes. Are they changed every 7 years?? Think aobut it. !!
moschoj - 5/25/2007 1:04 PM
Look at it this way- Look at the 100 year old cast iron radiators in homes. Are they changed every 7 years?? Think aobut it. !!
Fresh water mainfolds rarely need replacing. Salt water ones do. I live in Pa and I don't know one person to ever replace a manifold if the boat was always fresh water. I have been boating for 30 years. I know guys with 1965 Chris Crafts and they have the original manifolds. That said, if you do some salt water boating it is really cheap insurance. They are not very expensive anymore.
Hot-tanking removes any rust scale and sludge buildup that might be in the manifolds. I know the cooling system is 'abrasive' but there are no moving parts. A close inspection after taking off the ends has not disclosed and 'wear' on the inside of the manifolds or the risers. There is, of course, a light rust film, but the hot-tankiing takes it down to clean metal in no time. Next time I pull the manifolds I'll have them magnafluxed as a precaution. I suppose the 'flap' valve should be checked, but, it seems almost impossible to do without removing the risers which means the engine has to be removed. Any way to quick check the 'flap' without riser and engine removal?
__________________ NJM
If you are not the Lead Dog the scenery never changes.
21' SeaRay Cuddy w/Mercruiser 260 I/O
Replace Manifolds? 
