My second 1965 Dodge Dart.

Thank you. I have tried the cap, and it can be put in. But, the rubber seal is so hard that I can not push it far enough in so the locking tabs goes in and locks it. I have to find a new seal. At least I need to see if this is what is the issue.
Do you have a picture of yours, pm me if you have.

Bill
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This what I found here.