Joemommaaa;
I am wondering, while you had the starter removed, did you notice if your bolts were bent? The 168 tooth starters have an inherent problem. Due to their off set bolt pattern, the two bolts are at the rear most part of the mounting area on the block and therefore have little support in the case of a high compression motor. When the starter is engaged, it tries to pull away from the block mounting area. All the torque of the starter can actually lift the starter next to the block area and cause the starter to rock.

Powermaster has the answer as of this year, it is a brand new product. They make a mini starter with three bolt holes for the 168 tooth flex plate. This allows you to install a third bolt right next to the flex plate at the inward most area where the torque is the greatest. This totally prohibits the rocking motion of the starter and is a wonderful product.

However, a crack while cranking does sound like the problem is what Vegas mentioned. Good luck.