I dimpled the holes on the spar web for the countersunk rivets. Instead of using a dimple die to do the dimpling, I used a flush rivet to do the dimpling. I placed a flush rivet in the spar web hole that was countersunk undereath it on the thicker spar web. I placed the bucking bar under the spar to back up the rivet. I could do this because I had holes drilled in the bottom of the spar bucking bar to allow for the stem of the rivet that passes through the spar web. This allows the bucking bar to sit up against the backside of the spar web and give support for the dimpling process. Next, I use the hammer and bolt method to tap the rivet a few times from the flush side of the wing spar. This uses the shape of the flush rivet head to dimple the thin top spar web against the thicker, countersunk spar web beneath it.
Holes drilled in bottom of bucking bar to allow for rivet stems.
Pre-bucked rivet sitting proud on spar web.
Flush rivet after a few taps with the hammer and bolt.
