Initial Prep (Debur, Trim, Drill, and Cut)
9/12/22 – 1.9h
9/16/22 – 0.3h
9/17/22 – 2.1h
9/18/22 – 1.9h
9/26/22 – 1.1h
9/27/22 – 1.3h
9/28/22 – 4.4h
9/29/22 – 1.1h
10/2/22 – 5.7h
Like the outboard leading edges, the fuel tanks have a J-Channel along the top of the skin that gets match drilled to the skin. I cut the fuel tank J-Stiffeners from the 72” J-Channel stock for the outboard leading edges and tanks using a Dremel with a metal cutting disk, and completed the cut with a Dremel sanding barrel and disk. After trimming the J-Channels to the proper length, the inboard #40 “starter” hole is drilled according to the dimensions in the plans, and the rest of the holes are match-drilled to the skin from there.
[Dremel with Metal Cutting Disk, #40 Drill, Scotch Brite Flap Wheel, Hole and Edge Deburring Tools]

Final Prep (Dimple, Countersink, and Shape)
9/17/22 – 0.6h
9/18/22 – 1.1h
9/26/2 – 0.4h
9/30/22 - 3.4h
10/4/22 – 1.6h
10/5/22 – 0.6h
10/10/22 – 1.6h
The rivet holes where the fuel tank skins attach to the rear baffle are countersunk rather than dimpled due to the combined thickness of the skin and baffle. However, the skin is still only .032” thick, so extra care is in order to ensure the countersinks stay true to the center of the hole and don't go too deep. The plans specify that countersinks up to .005” too shallow are acceptable, and preferable to countersinks that are too deep. I set the countersink cage to .002” shallow, which worked for me to get close to flush, with a little margin for deburring. The nearly flush rivets can be shaved flush later.

The rear baffle gets cleco'd into place to provide a stable pilot hole for the countersinks. There is a note to leave every 10th hole uncountersunk to help with alignment of the baffle during the final assembly. I'm not totally convinced that will make a huge difference, but I deferred to the hundreds of tanks built before mine and followed the plans. The alignment holes will be countersunk and riveted after the tank is completely assembled and the sealant has cured.

Dimpling the skins takes a little extra care because there are a few different things going on. First, the holes for the rear-baffle were countersunk, so obviously none of those holes are dimpled. Similarly, the fuel tank drain flanges, which are attached to the outside of the skin, are provided with countersinks. The holes in the skin where the flanges attach are not countersunk. The trickiest “no-dimple” zone are the #8 screw holes on the inboard edge of the skin that align with the fuel tank attach brackets. Those holes, as well as the holes in the attach bracket shims, will be countersunk later in the tank assembly process. The plans include a note to “Dimple the screw holes in the fuel tank skins using the C-frame tool and a hammer rather than forming them with a rivet squeezer. This will result in "crisper", better looking skin dimples.”

The final step in this section is to countersink the fuel tank cap flanges for the dimples in the tank skins. I countersunk the flanges .007” deeper than flush, which worked well to get a good fit with the dimpled holes. The trickiest part of this step was clamping the fuel cap flange to the bench, which isn't particularly difficult.
[#40 Countersink Cutter, Microstop Countersink Cage, 22” C-Frame, Edge Forming Tool (Vise Grip Style), 22” C-Frame, #40 Standard Diameter Dimple Die, #40 Reduced Diameter Dimple Die, #8 Screw Dimple Die]

Assemble
9/15/22 – 2.4h
This section of the plans starts with a warm-up task to attach nut-plates to the Tank Attach Zees. Each bracket, with the exception of the most inboard one on each tank, gets three nut-plates that will align with the holes in the spar to help hold the fuel tanks in place.
[Pneumatic Squeezer, Flat Squeezer Set]