Mercruiser 7.4 Manifold repair

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A friend of mine is buying a boat with a 7.4L Mercruiser.  while going over the boat today he noticed a crack in one of the exhaust manifolds (from improper winterizing by the previous owner).  what is the proper way to repair it? I believe its a cast iron manifold, and as far as my friend knows its a mid-late 1980's mercruiser engine.i have a lot of experiance with stick, and some experiance with both tig and mig.If i were to do the repair I would have the manifold off the motor, and in the apprentice school.  I'll have acess to all three processes, and there is an airgas next door where i can buy consumables.  I will also have access to O/A if needed.thanks in advance
Reply:I can only summarize, as I have been taught to weld cast iron, but virtually never do so. Castweld is the guy to ask, if he is still online.Stick and O.A. can do it. It will need to be preheated, welded, then slow cooled. If you shortcut the process of slow cooling, cracks will form. Prep by cleaning and grooving the crack. Preheat the entire manifold evenly above 800 degrees then weld. Slow cool the thing for 12 hours back down to room temp. You could use dry sand to bury it, which will get you by.If you use O.A., you should get some cast iron compatible flux to use as well.City of L.A. Structural; Manual & Semi-Automatic;"Surely there is a mine for silver, and a place where gold is refined. Iron is taken from the earth, and copper is smelted from ore."Job 28:1,2Lincoln, Miller, Victor & ISV BibleDanny
Reply:Damage,You actually provided very little info on the manifolds.Is the engine fresh water or raw water cooled?Are these the original manifolds?If the manifolds are actually 20+ years old and raw water cooled (salt water in Boston), you've got about a snowball's chance in he11 of repairing a freeze crack.Best replace them both with OEM or Barr manifolds.Actually, if the engines are 20+ years old and raw water cooled, you're only going to (best case) get a few more years out of them.The reason I'm inclined to suspect that the manifolds are raw water cooled is because if the engines were fresh water cooled the manifolds would have contained an anti freeze solution and not water.   On a fresh water cooled engine, raw water will be found in the elbows but not in the exhaust manifolds themselves.Syncro 250 DX Dynasty 200 DXMM 251 w/30A SG XMT 304 w/714 Feeder & Optima PulserHH187Dialarc 250 AC/DCHypertherm PM 1250Smith, Harris, Victor O/ASmith and Thermco Gas MixersAccess to a full fab shop with CNC Plasma, Water Jet, etc.
Reply:Sundown, thanks! I don't believe that it was a raw water motor, i think it was a fresh water motor, Boat was registered in NH prior so i would assume it was on one of the lakes in NH.sorry for not providing more info, i will try to get some pics soon of the crack to give every one an idea of the issue.thanks for the replies
Reply:You said that he is buying it? If the deal hasn't been completed, have the current owner replace the part or reduce the price. if the deal is done, remove the manifold, prepare the crack by stop drilling the ends and grinding a bevel, heat it to dull red and weld it with Ni99 (if I remember correctly). Don't let it cool while welding.  Keep it hot till you are done. Then bury it in a dry sand pile or wrap it with welding blankets.That's one of several ways to fix it.edited to say that I could have just seconded the post by tanglediver but i didn't read the whole thread.Two turn tables and a microphone.
Reply:Believe it or not, but it makes a BIG difference whether the manifolds have been exposed to salt water, or whether this is a fresh water cooled engine.Basically, without explaining the details, if the manifolds were exposed to salt water for all those years, tig welding them is just about out of the question.Regardless, it's very difficult to recommend a process without seeing the manifold and what type crack we're talking about.Syncro 250 DX Dynasty 200 DXMM 251 w/30A SG XMT 304 w/714 Feeder & Optima PulserHH187Dialarc 250 AC/DCHypertherm PM 1250Smith, Harris, Victor O/ASmith and Thermco Gas MixersAccess to a full fab shop with CNC Plasma, Water Jet, etc.
Reply:Originally Posted by SundownIIIDamage,You actually provided very little info on the manifolds.Is the engine fresh water or raw water cooled?Are these the original manifolds?If the manifolds are actually 20+ years old and raw water cooled (salt water in Boston), you've got about a snowball's chance in he11 of repairing a freeze crack.Best replace them both with OEM or Barr manifolds.Actually, if the engines are 20+ years old and raw water cooled, you're only going to (best case) get a few more years out of them.The reason I'm inclined to suspect that the manifolds are raw water cooled is because if the engines were fresh water cooled the manifolds would have contained an anti freeze solution and not water.   On a fresh water cooled engine, raw water will be found in the elbows but not in the exhaust manifolds themselves.
Reply:Small hijack here... What does 'raw' water mean?
Reply:Originally Posted by daddySmall hijack here... What does 'raw' water mean?
Reply:Originally Posted by PangeaYou said that he is buying it? If the deal hasn't been completed, have the current owner replace the part or reduce the price. if the deal is done, remove the manifold, prepare the crack by stop drilling the ends and grinding a bevel, heat it to dull red and weld it with Ni99 (if I remember correctly). Don't let it cool while welding.  Keep it hot till you are done. Then bury it in a dry sand pile or wrap it with welding blankets.That's one of several ways to fix it.edited to say that I could have just seconded the post by tanglediver but i didn't read the whole thread.
Reply:Epoxy.
Reply:If there is a crack on the outside the inside is cracked too, it will leak water into the cylinder,just get a new aftermarket one