For grins I googled "Triumph primary case magnesium fire" and saw a lot of references to magnesium rims, and apparently BSA had a model with a magnesium primary case. Then I searched on "magnesium fire weld" ("weld" to capture welder, welded, and other variants). Thank you, internet, for again having directly conflicting answers! I got this hit in an article called "Welding of Magnesium-base Alloys" at http://www.keytometals.com/Article35.htm
"Magnesium castings, or wrought materials do not create a safety hazard since the possibility of fire caused by welding on these sections is very remote." Then the same search on the same page got hits from the Hobart Welders forum, WeldWeb, WT&T.com, and CNCZone talking about magnesium fires and not using water to put them out.

Rod, a drop of vinegar is so reactive with magnesium that it bubbles? Wow. So theoretically you could be eating lunch and drop a pickle or salad dressing, and melt your transmission?! (I can't get videos, just now.)