The shock just sticks up through a hole. Rather the rod portion of the shock sticks up through. There should be a rubber "donut" with a large metal washer and a nut on top. This would be seen from the engine compartment. If it's your passenger side shock that you are talking about, the top nut is under the overflow tank. Looking at the shock, there should be a rubber sleeve covering the upper portion of the shock. To get an idea of what parts are involved, go to Fastlane and look at the pictures of the Bilstein shocks. The rod that goes up and down has a small, fixed in place washer that a larger washer, then a rubber donut, rest on. The rubber sleeve covers this. If you shock is flopping around, either your top mount nut is loose or maybe this small washer broke and the larger washer fell down. Feel up towards the top and "see" where the large washer is. It should be about 1/2-3/4 of an inch from the top.

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1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member