pied_piper wrote:Agree with Mirwa. Many/most flute techs use very thin feeler material to check for equal seating 360° around the pad. This feeler material can be as thin as .001" or even .0005" if they are really particular. To put this into perspective, .0005 is less than the thickness of most human hairs.
After selecting a pad of the appropriate diameter and thickness, the procedure to install and seat each pad properly goes something like this:
1 Install pad in key cup and place light mark at key arm to indicate radial orientation
2 Mount key on flute body
3 close key and use feeler to inspect for places where pad contact with tone hole is heavy or light and mark the light spots
4 remove key from flute
5 remove pad from key cup
6 add or remove shims to adjust for heavy or light spots - some techs use partial shims for small areas
7 REPEAT above until the pad touches equally all around the tone hole.
8 REPEAT above for each pad on the flute...
There is MORE to the process than what I have described above, but that should give you an idea of the precision work required to repad a flute. This also does not describe the process required to regulate the interaction of various keys that must work together or close in unison with no lost motion.

What makes a pad seal securely for longer assuming it's a reasonable factory yamaha where QA is good-ish.