In the flap plans, there are a few callouts to fabricate tools and jigs to countersink and drill the trailing edge at a precise angle. While outright precision probably isn't necessary here, I decided to hop on the CAD and 3D print all these parts instead of fabricating them out of aluminum.

One challenge was machine countersinking the trailing edges. I noticed that Cleveland Aircraft Tools sold a jig to hold the trailing edge at the right angle to use a countersink cage with. $42 seemed like a lot for such a simple part, and I decided to draw one up and 3D print it also, which worked great.

While the flaps seemed fairly straight forward, there were a few new and interesting tidbits in the plans that haven't come up in previous parts. First was the use of duck-bill pliers to eliminate "pucker" in the ribs. I didn't actually own any duck-bill pliers by that name, but had a small pair of flat (non-grooved) surface pliers that I put some tape over to not damage the aluminum and straightened out the bend as best I could. It was pretty subtle to start with, but it came out slightly more flush so I suppose mission accomplished.

And "pucker removed"!

Next were the inboard leading edges that required doublers and a large nutplate. Pics still need to be deburred, but everything went together nicely.

The plans were not particularly clear on this, and I only discovered exactly which flanges should be dimpled in the top wing skin section. In short, do not dimple upper flange of the inboard 4 ribs! The doublers for the skin to reinforce the section that can be walked on are too think to dimple and will be machine countersunk instead.

Riveting the rear spar to the wing ribs was accomplished mostly with a squeezer. There were a few places where it was really challenging to fit a yoke without interfering with another rivet shop head or a flange. I found that I ended up cutting a small groove into a few shop heads, caused by the yoke dragging a corner against it. These didn't seem too deep, however, so I simply smoothed them out with a file.

The hardest part of the spar for me was the new inboard aileron bracket from the service bulletin. The rivets along the outboard flange of the bracket couldn't be squeezed because my squeezer would interfere with the bracket itself and couldn't be square. For these rivets I had to use the gun and bucking bar. For the inboard flange of the bracket, the neighboring shop heads interfered with even on offset rivet set. For these, I had to flip the rivets around so that the manufactured heads were on the inside of the spar. I couldn't quite square these up, and the shop heads ended up being

One challenge I've had is riveting flanges against a webbing or similar cases where the outer surface wants to lift up to create a gap between the two pieces of material. I made a few observations here and have found an approach that seems to work pretty well in a variety of circumstances.

Back-riveting

When able to back-rivet, I've found this to be the easiest as the back-riveting tool has a strong spring that compresses the pieces together before riveting.

Conventional riveting

When using the rivet gun and bucking bar, unless you press the sheets together with your bucking bar hand a gap will often form between the sheets. This has caused me a lot of early frustration, as once this happens the sheets become slightly deformed and even after drilling out the rivet it becomes even harder to get them compressed flush without a gap.

The technique that I've found easiest is to get a piece of rubber that has a hole punched in it just large enough to hang on the end of an unset rivet. You have to be careful on the thickness of the rubber; if it's too thick the bucking bar may not make contact with the rivet shop end. If the rivet is just slightly set with the rubber to compress the pieces, they will hold together. Then the rubber can be removed and the rivet fully set normally.

Squeezing

When rivets can be squeezed, I've found having the rubber insert works well here as well, but doesn't need to be removed to fully set the rivet. I've also found that the rubber pad tends to square up the squeezer to be orthogonal with the surface.