The 15" beadlock kit has 39 3/8" mounting holes, but use 5/16" bolts to avoid bind (& bent bolts) as the ring flexes. It is designed for nylock nuts and grade 5 bolts. All of our beadlock kits are of the within the lip design and must be welded by a competant certified welded, this is your butt we are talking about here so don't DIY if you learned to weld yesterday. The Steel rings are .25" (1/4) thick, and 8 are included in the kit.