Just be glad you are only missing that. Took my wheels off yesterday to install new tires and went to take the rear axle off and layed down to get a look at the pinch bolt. Was looking up there trying to figure out the size when I noticed there wasnt one. Apparently the previous owner never threadlocked it or just never put it back in. Got 2 new ones coming next week since the dealer made me order them in pairs but I'll at least have a spare.