Welding a cracked cast aluminum bell housing on a vintage Alfa … – TheFabricator.com

Detroit welder Josh Welton of Brown Dog Welding tackles a cracked cast aluminum bell housing on a vintage Alfa Romeo.  Photos by Josh Welton
Cast aluminum is a problematic material to weld. Part of that is due to the material itself, and part is due to how cast aluminum is made. “You should never judge a book by its cover” is as apt here as anywhere; it’s tricky because even a brand-new, “clean” casting can be a pain in the ass, while a 100-year-old dull and dirty piece of cast aluminum could weld smooth as silk.
My good friend Ralph is big into late ‘60s/early ‘70s Alfa Romeos. I wanna say he’s on his third resto/build just in the last couple of years? On one of these cars the cast aluminum bell housing developed a pretty significant crack after an unfortunate fall to concrete during the installation. Ralph wasn’t sure how readily available these parts were and asked if it could be fixed. Could it be? I was pretty sure the answer was yes, but that’s only part of the equation.
I remember a long while back working on another bell housing for a friend’s 1970 Charger. It was an oily mess, and at first glance my brain thought, “Man, this is gonna be a project!” I sprayed a little degreaser on it and wiped it down, heated it up with an oxyacetylene torch to burn out impurities, and sat down all geared up for battle. The fight never materialized. It welded beautifully and took me maybe 10 minutes to fix. On the other hand, I’ve also welded on brand-new castings that cackle and snort and devolve into a series of little black volcanoes popping up where the bead should be.
The crack in this Alfa housing was bad enough that it was gonna take a bit of finesse no matter the condition of the base metal.
If a part is rare, difficult to find, or superexpensive, then attempting a repair makes sense. But if it’s readily available for a reasonable price, what I charge for the repair might push the cost high enough to not make sense. It might be better just to buy another one in good condition. Ralph ended up finding a new one, but I told him to leave the cracked one with me and I’d see if I could fix it and document the process when I had the time.
Why is cast aluminum so hit or miss? Aluminum, no matter its form, is very susceptible to contamination. Any bit of dirt complicates the welding, and it typically needs to be completely ground out to lay down a clean bead. It can spread like the black plague if you try to weld over it, and then it becomes harder to fix.
The casting process adds issues you won’t find in other forms. It’s not going to be as dense as rolled or forged aluminum, so it’s a bit more porous by nature. It’s pretty common to hit air pockets and get some “popping” when TIG welding. Dirt can get trapped in pockets during the process as well. Because of this porous nature, if a casting has been exposed to oil or other fluids during use, chances are it’s seeped in, further complicating issues.
I wire-wheeled the surface on both sides of the housing’s crack, then hit it with the torch. A little oil came to the face on one end, but overall it looked pretty clean.
With any type of crack repair, whether it’s steel or aluminum, the first thing you need to find are the ends of the crack. If you start by grinding or putting any other kind of stress to the material, there is a high probability the crack will spread. Once you identify where the ends of the crack (or cracks) are, drill holes there. The round hole, with no sharp edges or stress risers, prevents the crack from growing.
The next step is to grind out the crack itself. If you attempt to weld over the crack, there’s a good chance the weld itself will crack, and that goes for any base material. The fractured edges are work-hardened and are brittle, so the material on the edges of the crack should be removed as well.
Cracked cast aluminum bell housing from a vintage Alfa Romeo. Cast aluminum is so problematic, and cracks only add to the welding challenge.
Depending on the size and placement of the crack, it might be best to tackle it in sections to maintain location and minimize warping. The crack on this bell housing was long and had several high-stress areas, or spots where the housing changed shape and thickness. After drilling out both ends, I also put a hole where the crack changed direction in the middle at a boss for a mounting bolt, just to make sure it couldn’t spiderweb up.
I used a cutoff wheel to slice a thin line all the way through, then a rotary file to remove excess material that might have been compromised. The rotary file also eats away any carbon that the cutoff wheel embedded. The crack terminated about a half-inch from a gusset on one end and just before the thick mounting surface at the other, so I didn’t have to fret about losing location like if a chunk had broken off. I did, however, start by putting down a thick tack near the boss to ward off any thought the opening would have of widening.
From the first arc it was obvious that this wasn’t going to be one of those quick and pretty jobs. The surface looked clean, but the puddle was a violent mess. I took it slow from the middle, working my way to either side running short beads, sanding and grinding them down, then running another bead on top, grinding it down, rinse, repeat. It’s a technique called buttering, and gradually there’s enough good material in there to lay a decent bead.
A piece of stainless steel, curved to fit the inside contour of the housing, made for a good backing bar to shield the root from the atmosphere. I didn’t go for 100% penetration from one side. Instead, I filled in the gap from the outside in before backgrinding the root and completing the joint from the inside.
Some people will bake the entire casting to cook out impurities before welding. Others will preheat the casting, peen the welds, and slowly cool it after welding to alleviate stress. I chose to build up my heat slowly through mixing TIG welding, grinding, and sanding, then dissipated it slowly the same way by mixing in grinding and sanding and welding. It’s not by the books, but it worked for me in this case.
I wire-wheeled the beads, but didn’t sand them down except for at the mating surface and the boss so the bolt would clear. There’s no point in blending it in unless necessary or you’re concerned about cosmetics.
And that was it. It took a couple of hours and a bit of experience, but all in all it turned out as well as could be expected. It’s ready to rock and roll for the next Alfa project!
The WELDER, formerly known as Practical Welding Today, is a showcase of the real people who make the products we use and work with every day. This magazine has served the welding community in North America well for more than 20 years.
Owner, Brown Dog Welding
Brown Dog Welding
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