There are two O-ring in the LNC2 cap. The large one you can see, and a small
one that seals around the shaft down the center of the cap. Several years
ago, I disassembled my fuel caps to try and polish them. I discovered that
the little O-ring had been leaking water -- quite a bit of white aluminum
corrosion was present between the two disks on both caps. Now I change the
small O-rings every year. You can get 50 or 100 of them from McMaster Carr
for a couple of dollars (buna-n, fuel proof).

Mike Reinath
LNC2 N3602M at 1018 Hrs