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I fitted a new rubber seal / breather baffle in the cap, after a few days I had a problem unscrewing the cap, when it finally gave in, I could see the seal had expanded (due to the ethanol I presume) locking the cap in position. Leaving it on the bench for a while it returned to it's original size, has anyone else encountered this problem with these after market seals ?
Thinking cap on, I decided to make one from alloy sheet, locating it between two thin cock gaskets. I am hoping the cock will stand up to the rotation friction, when fitting and removing the cap.

I made a new seal from viton rubber sheet. A few weeks later it had expanded. I reckon that cork is probably something that will not be affected by ethanol, but I'm probably wrong.

Make it out of leather, and when its glued on and set (that's if you need to glue it) grease the leather, have used leather as a sealing washer on dozens of fuel caps of many differing designs with complete success,
Cheers,
George

Colin , One would have thought Viton, to have been suitable, had the local factors not been closed due to the lockdown, it did cross my mind to try it, but will abandon that one now.
George, If the cork fails in use, your suggestion of a leather seal seems to be the way to go.
Thanks both.

Interesting. Made me go find out about Viton and ethanol and found this. Note the author is DuPont, the manufacturers of Viton, themselves.

Does Viton swell when exposed to ethanol ?

Source: http://www.dupontelastomers.com/autofoc ... ensAltFuel

While ethanol mixed with gasoline is known to lower engine/tailpipe emissions, previous work has shown that such blended fuels exhibit increased permeation through plastic and rubber.

This leads to the question - how do higher percentages of ethanol in fuel blends affect fuel system rubber components such as seals and hoses of fluoroelastomer?

A project was undertaken to document the performance of several common Viton® fluoroelastomers of varying fluorine contents tested in various blends of fuel and ethanol. The types of Viton® tested ranged from 64% fluorine content Viton® GLT-600S to 70.2% fluorine content Viton® GF-600S.

Two of the properties measured were volume swell changes and permeation rates. Figure 1 shows the volume swell results after an immersion of 168 hours at 40°C. Six different fuels were tested ranging from 100% Fuel C hydrocarbon test fuel to 100% ethanol.

The data shows that the lower fluorine types of Viton swell more than the higher fluorine types. Also the data shows that the highest swell occurs when CE-25, 25% ethanol in Fuel C, is used as the test fuel. This occurs with all six types of Viton® tested.

Interesting read, Thanks.