Almost four years ago, I riveted hinges to the bottom firewall flange, as shown in the plans. But ever since then, I've felt conflicted about these hinges, since I've seen multiple RV's that have broken hinge eyes in this area, and I've read accounts from many other folks that have the same problem. It seems that some combination of airflow and exhaust pulses causes the underside of the cowl to get beat up more than the rest, and the aluminum hinge is perhaps not quite strong enough to put up with that amount of abuse over the life of the airplane.
The final straw was when I ran into an acquaintance who's just recently started flying his RV-6A… when I asked him how the airplane was behaving, he said "Great, except every time I fly it I find another hinge eye broken off the bottom of the cowl!" It wasn't very long after that before I found myself drilling the rivets out of the lower firewall flange, and cutting some solid mounting strips out of 0.063 alclad to replace them.
These got match drilled and dimpled to fit the holes that previously secured the hinges, and then I gave them a slight bend to follow the upward taper of the cowl.
I'll use either nutplates or camlocs here – I haven't decided. There should be just enough room to squeeze the rivets that will attach them to the firewall. I am keeping the hinges along the vertical sides of the lower cowl, since they don't seem to suffer from the same malady as the bottom ones (and also because the engine mount would have to come off to replace them!).
