The crankcase is the engine housing: the crankshaft and bearings sit in it, and the oil is held in its cavity.
It should be replaced if it is cracked, the bearing seats are worn out, the threads are stripped, or oil leaks along the mating surfaces. Before assembly, flush the passages and replace the bearings and oil seals. If you have the old part, compare it with the new one for dimensions and mounting points before installation.
