I have an alloy skull tank lid and I run the Snappy fuel line kit.

I bought the kit 2nd hand so have never had the correct o ring for the lid.

I've tried various sizes in a kit I bought but the tank still leaks from the cap.

The hole in the cap has been sealed and the Snappy breather is breathing fine.

I'm about to order a nitrile O ring but thought I would get the size off the makers to get it right!