I'm not sure why some people are struggling with this and giving bad reviews. Mine came with an o-ring and fit perfectly. Once installed, it can only be removed by removing the valve cover and depressing the tab. If you try to take it off without removing the valve cover it will break the tab and wobble, that's how the original GM is designed, and it's designed not to come loose when you remove the oil filler cap. I think some of the bad reviews are from people who removed them after installation without removing the valve cover.
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