SonexBuilders.net

I Have a similar design cap using the same O ring. After a year in service noticed needed to tighten more to prevent leakage. (My cap uses a knob to screw to expand rather than a flip lever.) Replaced the original 70 durometer Buna-N O ring with one that was 50 durometer Buna-N. That worked great for a while but eventually swelled and made removal of cap difficult. Compared the original 70 durometer O ring to a new one from same batch and it had gotten harder. Installing new 70 durometer O ring brought sealing back to original without need to put more pressure on the plastic fuel tank's neck. (Using only 100 LL.) Replacing the O ring every year with new standard 70 durometer ones might be a good idea. Viton rings might be worth checking out as an option.

David A. Sonex TD

I received my gas cap today, thank you Peter! Fits like a glove and one of the nicest parts on the airplane!

Rick
Sonex 524