I've been reading up on possible knocking sound causes in other posts and have run across reference to "injector nailing". Can someone explain what happens to cause this and what it is?

Also, related to this, how do I "crack the hard lines" to each injector (one by one) to narrow down which injector might be nailing?

I'm still hoping for an explanation of flex-plate failure which one might miss-diagnose for rod-knocking...

Thanks.