fish', straight or slightly bent pipes should always pass a pipe cleaner unless they have a stinger or filter or some other intended blockage. I think bent pipes can be drilled correctly and well designed and still not pass a pipe cleaner, although I have a number that will. I use the various methods of jimmying through a pipe cleaner, but I believe that some airways turn enough to block a pipe cleaner. The bent pipes that do not pass pipe cleaners can simply be cleaned to their limit, then the stem removed from time to time to clean both halves, and they come up just fine. There may be Oom-Pauls that pass a pipe cleaner, but mine doesn't, and it smokes and cleans just fine.