Mine does that because i have an inch long, vertical split in the casing!!
Fear not though, as mine's a cast iron one and yours is posh alloy.
I believe mine is cracked because when i assembled the engine (Zetec) and box (non-posh type 9) the allen bolts i used were a knats member too long. It's tricky to explain without diagrams 'n' stuff, but when lying on the garage bench in a pool of its own oil, it seems easy to see why this happens....
...behind each of the four threaded holes on the g'box, it can be seen that the housing flares outward, so if the bolts are too long, they foul on the gearbox's sides and are forced outward.
I've since acquired a new 'box (£20 off eBay!), from which i intend to salvage a non-cracked maincase. This time, i'll use perfect-length bolts.
I've drunk lots of Peroni tonight, so i may be mumbling somewhat. Appologies there 😳.
Pikelet 😬