Bungo, I'm no expert on these issues. I just read teh forum here. There were a couple years right around 88-90 or so that had issues with the cases. Some will crack around the seal at the main shaft output (behind the stator) and some will crack where this guy's did, at the back by the oil pump. Earlier years had a thicker casting, I think, and later years that area was redesigned inside to be stronger. There's a thread on here somewhere with closeup pics of exactly what the problem is. Some google-fu may turn it up.

Going for the HD replacement cases or whole motor would certainly be the best solution.

 

In addition to the issues mentioned by Dr. Hess, these cases also were known to develop cracks between the lifter block openings.