Get yourself a small hooked pick from harbor freight, the small hook helps roll the o-ring on. As mentioned above the o-ring looks to be about half the size it needs to be, but it does stretch. I spray the o-rings with silicone before putting them on.
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And anytime you remove the cap you should replace that o-ring.
Another tip, I use a piece of air line tubing (like what comes with air bags) to seat the valve stem gasket whenever I remove a sensor.