Blocks are CNC machined from 6061-T6 aluminum. Feature 6 1/4" dowel pin holes for alignment when stacking the blocks. The center pin is 3/8-16 with a stainless steel thread insert. The center pin is 9/16", which is the bolt size for an Allen bolt. Blocks come with 2 1/2" long 3/8" Allens for the blocks, and 2 new 3/8" Allen bolts for the leaf springs. When ordering multiple pairs 12 dowel pins will be supplied.