Thanks Pete.

I haven't the foggiest what "section" means, though! 

Thanks both - I've finally got round to measuring the pipe. The external diameter of the groove in the silencer is about 29.5mm. The internal diameter of the exhaust is about 34.5mm. So I think that would support sizing of 27.4mm o'ring (given stretch) and thickness of o'ring at 3.1mm. Cheers

Hi

I just purchased one from a supplier on e-bay. Apparently provided it's a material called Viton it will withstand the temperature of the exhaust.

30mm x 3mm seems to be the size.

I have fitted it but not had a chance to run it yet as I'm in the process of changing the head gasket.

The seller is sealsrus - hope this helps

Thanks Hatrick1960 - I also found a supplier on the web called Simply Bearings - I bought a pair of Viton 27.5 x 3 for £5.50. They look to fit well, but I haven't had an opportunity to start her up yet to see whether they seal OK. Either way, it looks like there will be a much cheaper alternative to OEM. Cheers. Pete

That sounds like a bit of a stretch!

Very good idea to go for a Viton O-Ring. This material is the best choice for temperature resistance and non-ageing. Thank you for that tip.
On Ebay, there are multiple choices for the size. I will buy 28X3.5mm and 29x3mm to check. Price is 1.50 for each.
 
If somebody knows the size of the O-Ring for the carb (No5 in the picture), i could order this part in Viton as well
 

In the US there are Viton Oring kits available with an assortment of sizes, lots of sellers on eBay sell the same kit but usually for double the price, I'm sure some of them probably ship to the UK, or someone there has to sell similar kits? I used one on my exhaust pipe to silencer connection, I don't remember the size, I just found one from the kit that fit snug, it has been holding up so far.
www.harborfreight.com/180-piece-viton-o-ring-kit-67525.html

Back on topic, I used a 28mm x 3.5mm. I had to use a bit of copper slip to encourage the parts to fit together. I can't imagine any exhaust gasses getting past that...