Cracked Exhaust Manifold-1991 OMC Cobra-4.3L

I have cracks along the lower part of the outer exhaust manifolds from last owner's neglect to winterize. Can those be tig welded? What are the chances of an internal leak also?

Re: Cracked Exhaust Manifold-1991 OMC Cobra-4.3L

You shouldn't you would feel pretty stupid if you took it out and it started leaking and hydrolocks your motor to it's grave.

Re: Cracked Exhaust Manifold-1991 OMC Cobra-4.3L

Mine has been welded from before I bought it. It looks MIG welded not TIG but either way it is working fine for now.

Re: Cracked Exhaust Manifold-1991 OMC Cobra-4.3L

dougd59 said:

I have cracks along the lower part of the outer exhaust manifolds from last owner's neglect to winterize. Can those be tig welded? What are the chances of an internal leak also?

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if its cracked out the outside there is a real good chance its cracked inside also....welding out side cracks is a 50/50 thing. Ive heard of liquid weld being used and lasted for many years....I think new manifolds are your best bet "but" I would make sure you dont have any other issues with your engine (cracked block etc) from the poor winterization before you sink $400+ into new manifolds. If you are going to try welding the cracks pull the manifolds and inspect the inside for signs of water intrusion.......I personally think trying to cut corners and doing jiffy fixes lead to major issues.....$400 for manifolds vs 5,000 for a new engine if water drowns your engine...HMMMMMMMM
Denny

Re: Cracked Exhaust Manifold-1991 OMC Cobra-4.3L

Thanks for the reply. The engine is new. I replaced the old block, crank, and cam with a truck block and had the rebuild done professionally. The motor runs like a top! I'm having cooling issues. We went out last week to check everything out and noticed the leaking manifold. Pulled both to check them and they're both cracked. It looks like the lowest part of the manifolds still had water in them so they froze. There was anti-freeze in the system, but the block probably wasn't drained properly.
Where can you get manifolds for $400? I've even priced used ones for $500.

Re: Cracked Exhaust Manifold-1991 OMC Cobra-4.3L

I have a very good set of exhaust manifolds for a 4.3 v6 I will sell you for 400 if your still in need

Re: Cracked Exhaust Manifold-1991 OMC Cobra-4.3L

Howdy,

Can those be tig welded?

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Sure, you can weld many different metals with a TIG welder (GTAW) and weld cast iron using nickel or even bronze aluminum rod

What are the chances of an internal leak also?

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As good or better than the chances of an external leak.




It's VERY difficult to successfully weld or even braze cast iron unless you have the right furnace/oven to preheat the part.....and then correctly cool it to prevent cracking from uneven cooling.....

Even then they sometimes crack again. Because it takes specialized equipment etc, it's fairly expensive to get any big casting welded where they will give you any kind of guarantee.

It's just not worth it to try to weld manifolds since it's usually cheaper to buy new ones.

When you have jacketed manifolds (like nearly ALL marine exhaust manifolds) it's not possible to get inside to weld the internal cracks.

Using old, used or suspect manifolds on a "new" engine is just asking for a ruined engine.

I have a very good set of exhaust manifolds for a 4.3 v6 I will sell you for 400 if your still in need

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You should use "Private message" for making deals. For SALE bargaining in the open forum is not allowed on this site.



I would buy new ones from a reputable source....

Cheers,

Rick

Re: Cracked Exhaust Manifold-1991 OMC Cobra-4.3L

I was reading on the site tonight. I have a small crack in the port side exhaust manifold right near the drain petcock. I put the boat in the water this last weekend for a couple of hours. Some water would drip out of the crack. I'm trying to understand the inside crack comments vs outside. In looking at the part on ebay it looks like it is solid cast iron. Does exhaust pass thru on part and water another?

Any insight is appreciated. thx

Re: Cracked Exhaust Manifold-1991 OMC Cobra-4.3L

wellcraft185elite said:

I was reading on the site tonight. I have a small crack in the port side exhaust manifold right near the drain petcock. I put the boat in the water this last weekend for a couple of hours. Some water would drip out of the crack. I'm trying to understand the inside crack comments vs outside. In looking at the part on ebay it looks like it is solid cast iron. Does exhaust pass thru on part and water another?

Any insight is appreciated. thx

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Howdy,


Welcome to iboats......



This is a fairly old thread........It's really better to start a new thread with your question.

Here's what the Moderators are saying....

Announcement: -------------DO NOT reply to old threads or hi-jack new or old threads. Your reply WILL be DELETED. The old threads are for your information only when doing a search, and should not be resurrected. Click on this Announcement for further details. Don S (Moderator)

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now having said that, Exhaust manifolds for marine engines have water jackets so they do not get hot in the enclosed engine compartment.

The inside that is exposed to the exhaust gases can crack just like the outside. If they do crack, they will allow water to run into the exhaust passages. If the engine is not running and an exhaust valve is open, water leaking into that area would run directly into the cyl via that open exhaust valve.