How to measure a bottle hole to determine the correct bung size needed.
The best way to measure the internal dimension of your bottle neck hole is to use a piece of card as seen in the steps below.
Step 1: Grab the bottle, a piece of card and a pair of scissors.
Step 2: Make a T piece from the card bigger than the hole.
Step 3: Keep trimming down the sides of the T section.
Step 4: After a couple of snips the card will fit into the hole, ensure its a snug fit.
Step 5: Once it fits take it out and lay it on the table and measure the width with a ruler or other device, the measurement will the the min size of the bung, so in this example we would need a 19-21mm Bung