Then turn the expander a quarter round, and drive in the pin again.
The air does not pass the expander until after it has gone through the boiler already mentioned.
Finally remove the expander, and use the calking tool to bead the end.
Then force the expander into the leaky flue, bringing the shoulder well up against the end of the flue.
Loosen up and continue till you have turned the expander entirely around.