What size (diameter) was the rubber O- ring....
I think it goes on the filler cap...the filter should have a rubber ring already installed...around the perimeter of the base.
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You got lucky! I ordered 3 oil filters, still have two of them left, so I could take pictures
Please take a look at the pictures below and let me know if you need any additional information.
When you remove the oil drain cap, you will find similar metal washer attached in the picture - so I replaced that as-is with the new one. When I installed the oil filter, the thing that the oil filter gets attached to I inserted the rubber o-ring and then screwed the oil filter tight... not as tight as the guys at the factory did, but tight enough that there is no oil leak.
Hope this helps!