I have a late model series III 760 with this exact same problem and it has had me beat for ages. I have replaced - renewed everything, new bearings replaced pitman arm, nothing worked. I have had no trouble with disintegrating sieves or any of that stuff, just these stress cracks in the metalwork radiating out from the main crank bearing support.I bet Massey increased the speed of the shaker assembly on the later machines and put it just a little out of balance. We have added enough extra metal and welding rods to stop it but what is the next weak link in this particular chainIJIJIJIJ