If you use a Sullivan or Dubro, or Hayes tanks overtightening isn't really a problem. Where it comes into play is the sub-standard ARF tanks. You want the bung tight enough that the tubing doesn't move easily, and that you can't pull the bung out of the tank by hand.

There is very little pressure or vaccum on the fuel system. Any thing you can do with your lungs to induce a leak should be fixed, but more that that, I wouldn't worry about. The most likely culpurt in fuel tank air leaks is the clunk to brass tubing line. Some of the flexable tubing that is supplied with tanks today are not very reliable.

The two fuel tank problems I've had in the last four years are both related to ARF tanks. One split after over 100 flights, and on the other, the flexable line to the clunk broke in half. Once I have set up a Dubro or Sulivan tank, I've not had any problems with it.

Just make sure that the ends of the brass tubing are not sharp. Use a fine file and cut the sharp edge off first. Clean any filings out of the tubing and the assemble the tank.

Don