What is this o ring for?

[fivepointfivesi]

Im working on a 85 yamaha 200 engine. I recieved an upper engine kit and it didnt come with the cylinder o ring. My questions are, what is it used for? Is it a must to replace? there is a gasket so why the oring. And is this o ring a special size or can i use a non yamaha one. Or where can i just get the o ring quickly.

[Dirtcrasher]

If it's like Honda 250SX and 350X, I don't know why they show that oring at all. Why a gasket and oring? I feel the same way you do bud.....

[SWIGIN]

it keeps oil from traveling up between the sleeve and jug and making its way to the head gasket