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i have read through a lot of threads on this topic and thought i would provide some information to what i have found with some resources i have available to me. i have a 3969278NF casting in my 70 chevelle that i had tested where i work with a spectrometer to check the actual chemical composition of the center section.
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this is pretty well greek to me but the metallurgical engineer identified it as cast iron and suggested welding the tubes to the center section with Ni55 electrode. Now im not trying to tell anyone that this is the only way that this should be done since there is always differences in opinions when it comes to welding and the how to do this job properly. i just figured i would share this information and everyone can use it how they see fit. it could very well be that different casting numbers have different compositions so take this with a grain of salt and hopefully this helps.
 

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I've been telling people this for years. There are many people who claim that they are cast steel, but they are not. All of the '65 and later Chevy 8.2, 8.5 and 12 bolt rear ends have a cast iron center and must be welded as cast iron. This means that you can not drag out your MIG with steel wire in it and weld the tubes to the center casting. It might look like you welded it, but the steel wire will break off of the cast iron if it's stressed.
 

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I've been telling people this for years. There are many people who claim that they are cast steel, but they are not. All of the Chevy 8.2, 8.5 and 12 bolt rear ends have a cast iron center and must be welded as cast iron. This means that you can not drag out your MIG with steel wire in it and weld the tubes to the center casting. It might look like you welded it, but the steel wire will break off of the cast iron if it's stressed.
i was convinced by your posts that it was as well as the drill test i did to check the chips. but since i had the resources available i figured i could help you put this one to bed
 

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its also worth noting that he said Ni99 would work as well but his personal thought is that the Ni55 has greater strength and ductility and there for suggested it would be his choice to join the two pieces.
 

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its also worth noting that he said Ni99 would work as well but his personal thought is that the Ni55 has greater strength and ductility and there for suggested it would be his choice to join the two pieces.
Negative. Use as pure a nickel rod and TIG weld - use 99%. The centers are ductile iron castings. Ni55 would not be the first choice.

Read my posts on this thread: Welding axel tubes
 

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TIG and Silicone Bronze is also a very good option, even lower on residual weld stress/shrinkage scale and strong enough for the job at hand....it won't bust there before it breaks somewhere else.
Getting the weld joint area clean enough is the bigger part of the job. Needs careful grinder prep to remove contaminaints in the old press-fit tube-to-casting joint
 
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