From what I have heard others say about these o rings being hard to locate. I experienced the same on my 1978 cb750k. I have a o ring kit that had the right one, it is a 008 part no. with a inside dia. of 3/16 and outside dia. 5/16 What usually happens is these o rings either fall out of #2 carb bowl when rebuilding carb or the o ring just may need to be replaced. For my cb750 forum members I will send you the right one if you ask.