Trying to figure out what this seal/ring is for...

[XSBrendan]

Hello all I have recently introduced myself on the forum but have since forgotten my username and password so started a new account. I’m from Cincinnati I work three jobs and go to school for auto tech. My current project is a 1981 Yamaha XS 400 special I got for 150 from a friend. The bike sat for 10 years and I got it running, but noticed one of the rods was knocking.. while deconstructing the engine, I found a round cylindrical o ring and wondering what it is for. I have a new-old rotating assembly in the mail and would like to figure this out before I start putting things back together. It is in the center of the image provided.

Thanks a ton,
B

 

[BBS360]

Seals around one of the alignment dowels between the cylinders and head. The one with the oil passage.

 

[XSBrendan]

Seals around one of the alignment dowels between the cylinders and head. The one with the oil passage.

Thanks. Answers my question