You would think that with all of the advertisements\vendors\members that we would have a decent search engine... 10 years later, however, I have been searching for the past hour and I give up... can anyone tell me what the stock diameter is?

 

Inside diameter of the header outlet is 54.9 mm, outside diameter on the surface that the donut gasket attaches to is 64.9 mm. If you need the outside measurement with the donut gasket attached let me know.