Gearbox Shells

Is it the crack just between the bearing recesses? Has one or both of the bearings been turning in the shell?. Obvious by the polished surface caused by the outer race revolving. How slack are the bearing outers in the case? You can loctite the bearings in place if the case isn't worn. This is what loctite is ideal for.  

Asking again,where is the case cracked?

send a picture !

Do they crack anywhere else? No. If the bearing races are still firm in the case and have not worn the holes oval etc, loctite will stop any chance of them revolving. Have a look at the outside to see if the crack has propogated through (unlikely) It is only this very thin wall that cracks. Does it look as if the layshaft bearing has collapsed at some point in the past? Have you removed the bearings? if so look at the surface of the bores where the bearings sit, Does it look polished? a sure sign that the bearing race has been turning in the bore of the case. If the bores still show machining marks the bearing have been held firm.