Screw for fuel tank breather
This is a suitable replacement for the screw which goes through the middle of the fuel tank breather assembly by the filler neck. (It's number 16 in the parts diagram to the left)
This screw is usually rusted solid, so best cut or ground off and replaced with new. The original screw was M5 x35, but this is M5x40 with a 0.8mm pitch. Don't forget to order a new nut too.