The O'rings on the fuel cap of my 247 are failing quite often now, I believe that it me be due to E10 fuel. At the moment I'm just using basic rubber ones, can anyone recommend a type that will last a bit longer. Thanks

Stick an o ring in a container with e10 and see what happens !

Viton rubber is the go for petrol o rings

This subject is one that gets a lot of people arguing but is actually quite easy to research.

There are a lot of published O ring compatability charts, one that covers pretty much everything including ethanol is here: https://www.allorings.com/o-ring-compatibility

One problem is working out what the material is if you do not buy the O ring from a specialist supplier and get a "standard" O ring.  There really is no such thing as "just rubber".  As you can see there are two types of Viton so the above comment is correct for one type not the other.

The shift from E5 to E10 is not really that significant so if the bike was OK with 5% it is probably OK with 10%.  The compatability charts are for 100% of the selected chemical.  People also overlook the many other chemicals in petrol because there is a lot of nonsense being promoted about E10 by the same sorts of folk that promoted nonsense about the removal of lead from petrol.

O rings are generally pennies but the problem is where to buy just one and whether or not you believe the eBay seller or bike shop as to what material it really is.

I have bought O-rings from Simply Bearings.  They do very detailed descriptions of the various materials (there seems to be about 7 different types of material) for zillions  of O-rings of all shapes and sizes from less than 2mm to 600mm and beyond with details of various chemical resistances. 

The link is:-

Choose Nitrile, Viton, EPDM, O-Rings Seals by chemical resistance and temperatur (simplybearings.co.uk)

They do not seem to sell single o-rings but sell usually a pack of 2. Fine for me because I needed 2 for two identical bikes.

 

 

I used viton rings in my 348 tank cap. It becomes a very tight fit almost too tight to get out. Luckily I was able to machine the grooves a little bit deeper  to loosen it off.