Are you guys checking them just from the top? As you can see in Billman's post, the retainer on the right doesnt look cracked from the top, just the bottom of it, which is hard to see with it installed in the head. I see why Billman is leaving his, but as most ap1's are not owned by the original owner, the history of the car is never certain. To be 100% certain that they are not cracked, they need to be taken off. If they are taken off they might as well be replaced, especially when you can get the kit for around $50 shipped.

 

The damage to the right retainer is instantly visible from the top. It is plain as day to see when installed.

Bottom line, I am confident in over 600 cars my findings of what I call good retainers, just from looking at the top.