Tanning Bed Ball Studs Strut Balls ETS Sunvision and Sunquest beds used to have 13mm ball studs on the beds for struts. They stopped using them ABOUT 1995 (depends on the exact model) and all current struts use 10mm ball studs. The 13mm measure about 1/2" about the size of a AA battery, and the 10mm measure about 3/8" (just bigger than an average pencil) . If you are putting new struts/shocks/gas springs on your bed, you may need to get the smaller ball studs. They are easy to change, an adjustible wrench and 5 minutes. There are 4 ball studs on a tanning bed (one for each end of two struts to snap onto) and this auction is for a set of four.