I'm repairing a set of aluminum flaps with some cracks around the rivets on
the trailing edge. Take a look at the flap trailing edge of a later model
172 and you will see a neat 3-layer sandwich. The older 172's did not have
this feature, a thick beaded stiffener in the middle which stiffens and
strengthens the flap's trailing edge. The two outer layers of aluminum, top
and bottom, are the corrugated skins. In
the middle of the sandwich, between the two corrugated skins, is a 1/8"
thick aluminum extrusion about a half inch wide with a round bead protruding
from the extreme trailing edge.

So we bought some of this beaded extrusion and are riveting it into our
flaps. And yes, it's PMA'd. Google McFarlane Aviation and flap bulb.
The problem is this: how do you duplicate the
rivet holes in the extrusion without wallowing out the holes already
factory-drilled in the .032" aluminum skins? A strap duplicator is
impractical and so is a center finder. The fit of the extrusion inside the
skins is too
tight to use a strap duplicator and the skins are too thin to use a center
punch.

I was looking around in the RV groups for a little guidance but didn't find
anything much, other than a neat article by John Ammeter about riveting in
tight places.

[Later] No problem. A friend came through with a spotting punch. It has
a tiny little prick reminding me of Janu...., well no, I won't go there.

Now a thing or two on the subject of RAH. Don't let the trolls take over
here. People like Janulewicz and Morgans who do not build and only care to
disrupt and screech and stink the place up like the yahoos in Gulliver's
Travels and exhibit their stunted pudenda. Call their hands when they try
to take the floor or screech for attention. This forum can be an excellent
resource for young builders coming along with their projects, and many of us
can learn from each other. Don't let them distract from the subject or
publish bogus legal advice or whine about their backaches or their morphine
addictions or their deviant appetite for fudgepacker lubes. Stick to the
subject of homebuilding aircraft. I'll try to stick with it too.