Earlier I had enlarged 2 holes at the top of the firewall recess for 2 platenuts, but I did not have the right part. I received the MS21051-L3 platenuts and now was time to install them. I used two screws to secure each platenut to the firewall and used the rivet holes in the platenuts to match drill through the SS firewall and stringer. I removed the platenuts and deburred the holes. Next I machine countersunk the 4 holes for AN426AD3-6 rivets. I secured the platenuts with screws and back riveted them to the stringer
