The Kohlers are built strong and are easy to rebuild and work on. The 12,14 and 16 share the same block and it seems to me that the 12's seem to hold up better perhaps because they do not produce as much heat and have more metal left in the block.
Brian, I hate to bust your bubble, but the 12 Hp K301 is not the same block as a 14 Hp K341, the head bolt pattern is a dead giveaway. Also, the 12 Hp K301 shares the same block as the 10 Hp K241. I have never owned a 16 Hp Kohler, so I am not even going to try to guess there. Just wanted to clear up Kohler confusion.