petrol cap gasket failure on brand new outboard

[charliehows]

bought a 2hp 2 stroke yamaha outboard last week, second tank of gas i notice the rubber gasket inside the fuel cap had swollen up and broken. I went to Clarks rubber and the (expletive deleted)store assistant there recommended i use this neoprene to replace it with - bought a bit of sheet and cut it out, fitted it, checked it next day, swollen up and broken f#%!!*&%!...went back, they reckoned black nitrile is the go - but - none in stock...f%#!!$%!
no.1 Im mystified how a brand new engine could be fitted with a part made from unsuitable material
no.2 where do i get something to make up a gasket - at the moment i've replaced it with a couple of layers of gasket paper with grease rubbed in, probably work quite well in the interim but eventually i'd like to get a rubberish sort of gasket in there
cheers

[GordMay]

Rubber Characteristics: A & D Rubber Company | Materials