Disco 2-2001 Front drive shaft fell out, cracked transmission housing.
 

It seems generally accepted that this occurrence is often classified as a catastrophic event. So I believeI got a bit lucky when the rear UJ broke and the gearbox sump flange took a good smack from the prop shaft end. The result was that a small piece of the transmission sump flange was damaged, the rubber seal was also damaged and the gearbox oil leaked out.Upon removal of the transmission sump it was revealed that a small crack of about 15-20mm was evident from the edge of the flange of the flange running up the transmission housing. (See pics). Question is, how can this crack and seal flange be easily repaired with incurring the cost of removing the entire transmission housing. I think that the new sump seal will seal against the flange but I suspect fluid will leak out the small crack above the flange? Please could any body advise of the best aluminium crack repair option. Many thanks to all.

The proper way would be to replace it. If it were me is try and get that aluminum welded where it sits , can't hurt to try.

After having the case welded, then go buy a new drive shaft, British Parts of Utah has a good strong , American made from Utah, on sale.
In the future grease all 7 fittings at each oil change to prevent this from happening again.

Weld. Any type of repair filler will fail at some point from heat/cold cycles, pressure, and vibration. This is not like caulking a shower drain. They may want to drill a tiny hole at the end of the crack as well to prevent spread. All of this should be done with valve body covered and taped off, grit is no friend to a transmission. You could put a solvent fix, like SeaFoam tranny treatment (or just clean tranny fluid), in a spray bottle and squirt off the valve body area to flush out grit, once you are done.

you may not think it now but your one of the lucky ones, a little rtv on the gasket and it should seal up at least you didnt have to go for a new tranny

There's no pressure in the fluid against that area so it would be a minor leak at most, but having a crack like that is a concern as it may grow over time. To properly fix that, it would need to be cut out and them welded back up. I'm not sure how many places would be willing to do that with the tranny in the truck. You might be able to get away with blasting the area clean with brake cleaner, grinding it out a little bit and slapping some JB Weld or a similar epoxy type product over the crack. Clean up the mating surface and put a dab of sealer on it when you put the new gasket in and keep an eye on the crack. If it grows, replace the tranny.

always drill a hole at the end of the crack that will keep it from continuing.

The trick is finding the actual end of the crack while under the truck. Clean the area of all gunk and oxidation and do your best.

Man did you dodge a bullet.

Drill the end of the crack to stop it, then weld it or at the very least use something like Alumiweld to fill the crack.

Well I will chime in with an alternate view here. That are is not under pressure during operation and in fact the transmission pan isn't even filled to that high a level. I would put some RTV on the gasket and let it go, if it leaks then you can go at the more extensive repair but I doubt it would be need.

If having the crack really bothers you but you do not want to go for the full welding, you could clean it with some brake cleaner, let it dry, then apply some of the loctite used for already assembled fasteners. That chemical is designed to flow in to cracks via capillary action and although not designed as a sealer it does reach a firm yet flexible set.

Good luck