You know, I replaced my cotters recently and managed to strip the threads on the news ones almost immediately, applying almost no torque. A friend of mine had picked them up for me at a shop owned by a friend of hers - she didn't even ask me to pay her back for them - so I certainly didn't want to complain about the quality of them. The fact remains, however, that they were able to withstand very, very little torque.
So I looked around before replacing them, and in particular I looked
here. It turns out that the cotters I wrecked looked like what is sold here as "Grade D" - and it looks to me like yours are, too. I ordered a pair of the "Grade B," the highest grade in stock at the moment, and the difference was night and day.