James,
One thing we should find out is if the bellhousing was ever
heat-treated for strength. I don't know if the OEM units were or
weren't. Regardless, the temperatures involved in welding aluminum will
kill the heat-treating in the HAZ (heat affected zone). And because of
it's higher thermal conductivity, the HAZ in an aluminum casting will be
considerably larger than in a ferrous material. This is why aluminum
welds are considerably weaker than a heat-treated parent material (T6,
etc.). If anyone in the group knows if the bellhousings were
heat-treated at the factory, please let us know.
To your original question, I'd just make sure that you take it to a
competent weldor and ask if he pre-heats the casting prior to welding
and uses a back-purge. If he says 'no', then turn around and walk out.
If he says 'yes', he's a keeper, and will probably know what kind of
filler to use (or rather what filler works best for him).
You can get away without preheating aluminum if its small stuff like
plate, and thin-wall tube, but on big, load-bearing castings and
forgings, you want to make sure the whole piece is up to temp before
welding, or else you run the risk of warpage and more cracking when the
HAZ cools.
james wrote:
>Has anyone ever cracked a factory 4 speed bellhousing? I was starting to put
>my engine back into the car today, and when it came time to bolt the
>bellhousing back to the engine, I got about half way done when I noticed that
>it was cracked. The thin area below the clutch lever hole is completely
>broken through on the transmission end. There is another crack, tracing
>around the transmission area from the linkage hole up to about the middle of
>the bellhousing. The cracks are not on the machined parts of the bellhousing
>but where the transmission mounting area joins the main casting that covers
>the clutch.
>
>I'm not sure when this happened but I have two guesses. First, and least
>likely IMO, is that it was cracked by the shock of the engine destroying
>itself while cruising at 4500 RPM on the highway. Second, and more likely
>IMO, was that it was the shock of driving hard with the Buick 4speed and a
>3.93 rear end although if this were the problem I expect that there would be
>more accounts of failures. Any ideas?
>
>I'm going to have it welded back up and have the clutch linkage area filled in
>as well, I will then drill a hole for my flex lines to pass through. I intend
>to have it professionally welded since I don't have a lot of experience
>welding Al, but does anyone know what alloy of rod to use to reweld it?
>
>James Nazarian
>71B Tourer
>71BGT V8
>85 Dodge Ram
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