They were the same snap-rings from my stock set-up, I just swapped them over to the kit. I even made sure to check they were aligned and not too loose. Shoulda just got new ones, it would have been a quick trip to treats.
My guess is one broke and it somehow threw the other out of whack, not really sure. The other one was no-where to be found, it's probably stuck in my case and in the head and piston.