The trial fit after riveting all the hinges the fit still looks good my fingers are pointing to the areas that will receive nutplates after being drilled to #19 … the hinge tab on both sides will also be drilled and receive nutplates.
Now that the cowling sections have been fitted to the fuselage and the hinges riveted onto the cowling sections, the lower cowling needs to receive eight nutplates … six on the front of the cowling and two on either side for the side hinge tabs. The process begins with drilling the six #40 holes on the front of the cowling out to #19 with the upper cowling in place.
Drilling one of the six holes on the front of the cowling out to #19.
After the holes on the front of the cowling were enlarged to #19, the upper cowling is removed so nutplates can be installed on the lower cowling. A screw with a pile of washers on it was used to secure the nutplates to the cowling so the #40 rivet holes for the nutplates could be drilled.
Drilling the #40 rivet holes for the nutplate that will secure the tab for the left side hinge. The nutplate is being held secure by a screw and a pile of washers from the backside of the cowling.
Once all the #40 holes are drilled for the eight nutplates, the rivet holes for all the nutplates need to be countersunk for flush rivets. The two nutplates for the hinge tabs were done first because the countersink cage could not be used on the remaining six holes because of the joggle on the lower cowling prevented the countersink cage from sitting flat over the rivet holes. I opted to just remove the cage and hand turn the drill chuck. This allowed for good control on how deep the countersink bit was going. Yes, this took a few minutes per rivet to accomplish but the depth of each countersink was very easy to control. Also, the nutplate was used as a guide for the tip on the countersink so there was no drifting of the countersink bit.
Because of the joggle that can be seen in the photo, the countersink bit was turned by hand so the countersinking could be controlled. This is because the joggle prevented the countersink cage from sitting flat.
When riveting the front nutplates, I noticed the fiberglass was thicker in the area of the four inboard nutplates … so a slightly longer AN426AD3-4.5 rivet was used in the place of the typical AN426AD3-4 rivets.
Riveting one of the forward nutplates onto the joggle on the lower cowling using a slightly longer AN426AD3-4.5 rivet.
All the nutplates are now installed on the lower cowling.