Modern Vespa

I put a new rubber gas cap gasket on my 1963 Allstate and it fit perfectly. After going through a tank of gas I opened the gas cap to refill the tank. The rubber gasket fell out of the cap. When trying to put it back, I noticed the rubber gasket was much too big to fit into the cap. It had swollen most likely due to the gas fumes. It took four hands to put the enlarged rubber gasket back in the cap and then close it. The next time the cap was opened, the gasket falls out again. Should the rubber gasket be glued in the cap initially? If so, what glue? If the swollen gasket were to dry out, would it shrunk back to normal. If so, I could then carry a spare gasket to use for fill ups. There must be a simple solution to this problem. Any suggestions are most welcome.

I went through a number of these with the same issues. I bought one similar to this from SIP and it's been on for about 3 years and does not swell. In fact it's been swell! The lip is kind of handy too, although I'm still to find a solution to the problem of not being able to see when the fuel is nearly at the top!

http://www.sip-scootershop.com/en/products/gasket+tank+cap+for+tank+_92290000

I've seen some shrink but most swell a little, in general it's just enough that it's holding in the cap. try a different brand, maybe you bought some cheap stuff?

Try a red tank gasket. Like the red float needles, they seem to be less prone to swelling and shrinkage. The one Ginch uses looks like the best solution though.

I had the same problem, so I found some sheet rubber that was fuel tolerant and made my own. A lot of the rubber we get through the various vendors are from India that seem to supply the Stella brand. The quality just is not there.

I ordered the SIP fuel cap rubber gasket that Ginch mentioned. In the meantime, I had a spare new rubber gasket and put it in the gas cap. It fit perfectly. I then took the old gasket that had swollen due to the gas fumes and would not fit in the cap and let this swollen gasket air out for a day or so. On my next fill up, I tried this gasket again. It had shrunk and fit perfectly. So I will alternate at fill ups between the two rubber gaskets and should not have any problems with the fit. I still am looking forward to using the SIP gasket.