Gas Welding Aluminum

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I'm sure this has been asked before, but here goes: I have an old Offenhauser intake manifold that has a crack about 6" long.  I understand that welding aluminum with O/A requires flux, but how do I determine what filler to use?TIAJack JacksonCulver City, SoCal
Reply:Acording to one welding book, BAIsi, Aluminum silicon used along with AWS type number 1 flux is what  you want to use.  But be careful about overheating.Common sense in an uncommon degree is what the world calls wisdom.
Reply:Thanks!!!  I was beginning to wonder if anyone was here!!!
Reply:One more question:  What about pre- and post-heating?  Thanks...
Reply:i know some people preheat al when mig or tig welding thick pieces, being that the gas welding has a longer heat up time then electric arc welding, Preheat should not be neededHow thick is your metal?StangnetShop Full Of Stuff. Joey
Reply:I'm not sure about post heating, but you'll want to watch your puddle at the end of the weld.  From the molten state to a solid, aluminum contracts 6%.  That is where cracking mostly happens.  Try to taper the puddle size down before cutting the heat.Common sense in an uncommon degree is what the world calls wisdom.
Reply:Thanks for the extended replies.  How thick is your metal?
Reply:bead blasting will do an excellant job for pre weld prep, i would wipe it down it acetone or carb cleaner to remove final impurities. I use scotchbrite wheels to clean all of my metals, i don't weld with aluminum yet but i have used one to clean a piece i've brazed, but i use it on all my tig steel pieces too, wipe down with carb cleaner or other solvent on hand and it's all setStangnetShop Full Of Stuff. Joey
Reply:Awesome.  Thanks for the tips!!  When I get around to welding this piece, I'll post before and after photos...
Reply:Another thing I do sometimes when worried about the end of a bead is to tack a run-off tab to the work.  Not sure you want to do this on a manifold but it is a suggestion to insure the start/end of a weld is not contaminated or cracked.Get it hot and hit it hard...
Reply:Can you describe a run-off tab?  I've never heard of one.  I'm not horribly concerned with the appearance of the manifold, more with functionality.Thanks!!!
Reply:run off tab is just another piece of the metal tacked on the end of where the bead is so you can continue the smooth bead on the whole piece and finish it on the runoff tabStangnetShop Full Of Stuff. Joey
Reply:Gotcha, thanks!!  In the case of a casting, would any piece of aluminum do, or would I have to use a piece of similar casting?
Reply:most likely you would be allright with another piece of aluminumbtw, you're logo looks good!!StangnetShop Full Of Stuff. Joey
Reply:most likely you would be allright with another piece of aluminum
Reply:those were awesome, thanks for sharing them!!StangnetShop Full Of Stuff. Joey