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Cracked Head

 
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Peter  
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PostPosted: Wed Aug 08, 2001 5:43 am    Post subject: Reply with quote

I found two small cracks under the intake valve seat of the No. 2 cylinder on my 931 head. The car, when I got it, had the classic blown headgasket problems 1) oil and anti-freeze mix, 2) steam out the exhaust pipe, 3) bubling over radiator overflow container.
Although I beleive that this head has now become a paper weight, has anyone ever repaired a crack in their head, such as using epoxy etc?
-Peter A. Holiat
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larso  
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PostPosted: Wed Aug 08, 2001 9:39 am    Post subject: Reply with quote

hey, i got 14 bucks out of my crapped 931 head, they pay 45 cents a pound for a luminum, i think they pay way more in the states/lars
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Dave  
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PostPosted: Thu Aug 09, 2001 2:25 am    Post subject: Reply with quote

You could try using JB weld to fill in the cracks But I dont know what will happen
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Rick MacLaren  
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PostPosted: Thu Aug 09, 2001 2:41 am    Post subject: Reply with quote

That's a really timely question. I was speaking with my father about epoxy's and repairs to metals and he said that they used an epoxy for high pressures on steam turbines at Hydro plants for years. I used an epoxy based item called I think 'aluminol' for the inside of my wastegate.

Loctite makes products that are real specialty products.

http://www.loctite.com/selector/

The website lets you set parameters for the specific application. Give it a try.


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Peter  
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PostPosted: Thu Aug 09, 2001 5:21 am    Post subject: Reply with quote

I have JB Weld and I will gladly try it. JB Weld's web site says it holds up to 500 F. I couldn't find a similar product on the Loctite site but I am sure they have a similar epoxy.
I have a spare head that I am using but will JB Weld the cracked head. Hopefully I will not need it. As to testing the JB Weld, or any other epoxy, the best test is to install it on the engine and drive it. Unfortunately this is not easy. Pressure testing does not take into account the heat and other stresses that actually affect the head in real world conditions. I have yet to find a shop that heats their heads before they pressure test.
-Peter
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Dave  
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PostPosted: Thu Aug 09, 2001 7:21 am    Post subject: Reply with quote

This guy I know used JB Weld on his Porsche 924S. He had a chunk mising out of the block right on the flange where the head and block meet. He hanst had any problems with it at all. And he drives it every day
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larso  
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PostPosted: Thu Aug 09, 2001 9:53 am    Post subject: Reply with quote

Just a note that cracks are really hard to fill...JB weld doesn't stick well unless you are filling in chunks as Dave has told us...cracks are really hard to fill without heat like for example welding..

How are you gonna get the JB weld in the crack, it's a little chalky sorta. and isn't tacky...
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Dave  
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PostPosted: Thu Aug 09, 2001 3:47 pm    Post subject: Reply with quote

I dont know????? You could have some one weld the cracks in the head with alumium rods and then file it down. Its alot of work and it still might not be right. ALso the head might be worped
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wdb  
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PostPosted: Tue Aug 14, 2001 9:10 am    Post subject: Reply with quote

I asked a professor about repairing cracked heads and this is what he said.
Anyway, the real problem with the remelting of an aluminium alloy is that any crack will be faced on both its sides with aluminium oxide. This does not re-melt until 2050 C. Also, the oxide is chemically very stable. Thus this is permanent damage, and after re-melting will be likely to remain in place, and re-solidify as a crack. (Other alloy systems such as cast iron for instance would not have a problem in this way). There is no known way to remove this oxide.
Thus I am concerned that the re-melting would not remove the original cause of the failure. It is just possible that the sides of the crack may be closer together than originally, and so might leak less, but this would be purely fortuitous. It is possible that the crack may open more as a result of all your efforts. It will certainly not be sealed nor repaired.

Even welding of a crack in aluminium alloys often fails, and only works if the welder rabbles the liquid melt pool so as to disturb and so mechanically break up the oxide film. The welder will be lucky if all this effort does not introduce additional oxide films that re-create the problem.

Thus I think your idea is inventive, but I regret that there are fundamental reasons why aluminium alloys are resistant to being repaired.
John Campbell

Professor of Casting Technology
IRC in Materials, University of Birmingham, B15 2TT UK



[ This Message was edited by: wdb on 2001-08-14 16:08 ]
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larso  
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PostPosted: Tue Aug 14, 2001 9:28 am    Post subject: Reply with quote

that guy knows what hes talking about but says thus too many times.

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Dave  
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PostPosted: Tue Aug 14, 2001 2:32 pm    Post subject: Reply with quote

Just try to fix it Weld it. If it doesnt work at least you tryed. Or just spend some $$$$$ and get a new one
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