Fuselage: Firewall. Dimpling and countersinking.

The stainless steel firewall has two cutouts with AD3-sized rivet holes along their perimeters.  The plans specifically state not to dimple those 8 rivet holes on each side.  The callout only references the right side, though the text clearly states both sides (also, you're going to want a small diameter 1/8" female dimple die to avoid distorting the curved flanges on the firewall parts).

That didn't stop me from dimpling the left side anyway.  Whoops.

I skipped ahead in the plans to see what sort of grief and/or ca$h outlay I just caused myself.  Turns out that the vents go there (see page 28-10) and their parts can be easy dimpled or countersunk to accept my special dimples.  So all's well.

The firewall includes a fairly beefy frame as part of supporting the fan up front.  Each one of the angles that compose the frame has to be countersunk where they meet the dimpled firewall.  Here's a layup to find the holes that do not get countersunk.  Covering those holes with tape helps avoid errors.

Turns out there are 196 holes (subject to counting error) that need to be countersink, to accept a 0.025" thick stainless part, some 85%+ of which are for AD4 rivets.  That's a lot of work.  And though the angles are thick, the holes still come down to a knife edge when countersunk, so be sure to provide additional thickness to retain your countersink bit's pilot to avoid elongating holes.  I used a thick aluminum angle with various holes drilled in it so I could keep my work from moving around.

And here are the 196-some countersinks, completed over three days in 2 hour sessions.  Glad that's done.