14 We only have two holes drilled at this point, but mount the seat back to the bracket with four Grade 8 bolts as well. Using button-head bolts here can make life a little easier on your back. You will also notice the use of large fender washers. These are critical to provide a more widely distributed clamping load on the aluminum sheetmetal the seat is constructed from to minimize the chance of it ripping loose. All we had on hand were Grade 5 fender washers, so they will be replaced with hardened fender washers on final assembly.