Water Rocket 2-Stage Mechanism by air.command

Step 10:

Assuming the outside diameter of your tubing is exactly 5mm, drill a 5mm hole straight through the top of the cap. It does not need to be exactly on center.

NOTE: The tube needs to be a snug fit in this hole. You should not be able to slide the cap freely on the tube. If the hole is too big, get a new cap and drill a smaller hole.