Since installing Al's Upper A Arms and Moog grease-able lower ball joints, I wanted a method of keeping them clean until it was time to grease them agian. I went to the auto part store and found these.
Dorman - Autograde 7/32 In. Rubber Black Vacuum Cap
http://shop.advanceautoparts.com/p/dorman-autograde-7-32-in.-rubber-black-vacuum-cap-47392/5019761-P?vehicleIdFromReq=303649&isAllVehicle=false&vehicleIdSearch=303649&searchTerm=47392&showTitle=
They came as a 6 pack. They were too long for a grease zerk, so I used tin snips and trimmed 1/4 inch off the bottom, and they fit nicely. I will keep checking them and let you know if they fall off within the next 5,000 miles.

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Phil
2005 131k 3.7 Gasser (Lifted-CRD Springs, 4.10 Gears, F+R DTT, Teraflex 2wd Low, Skidrow/Mopar Skids, Rock Lizard Super Skinks, Goodyear Wrangler Authority 31x10.5-15)
2012 Triumph Tiger (TKC 80 Front Tire Heidnau K60 Rear, Crash Bars, Skid Plate, Bark Busters, Pelican Case Top Box)