The plans call for making shims as required prior to drilling the ribs to the aft spar on the flap. The last rivet hole in the ribs is not drilled until a later step, so I used the hand clamps to hold the ribs tight to the skin and get a good idea of what thickness shims would be required. First rib from the left fit nice and tight to the spar, so no shim was required. I tried several thicknesses of stock (.025, .030, .040) and the other three seemed good with .025 shims. I made the three shims and used several clamps to hold the rib, skin, and shim in place for drilling. I marked each shim and rib with a number so the shims would go back into the same position for final assembly. Got all drilled and cleco'd.
