I fit the lower right longeron. Here is the forward end showing how it tucks around the lower engine mount. I also drilled the bottom holes out to #30 to see what kind of edge distance I’d end up with on the mount. You can sort of tell here that I biased the holes 1/16″ inboard of the center point on the longerons to give me slightly more edge distance on the steel.
Here you can see that I ended up with just over 12/32″ on the engine mount. This is perfect as it means I more than 2d on both the mount and the longerons.
I first match drilled the mount to #30.
I then opened up each hole with a #15 bit (0.180″) before reaming it with a 3/16″ (0.1875) reamer. I’m trying to only use reamers for all of the bolt holes. They make perfectly circular holes that allow the bolts to slip in with virtually no play. Here I’ve dropped some AN3 bolts in place to keep things aligned. btw, it’s not an optical illusion, the bolts are intentionally spaced unevenly on the plans.
I repeated the procedure on the left side. Edge distances came out great here as well with over 2d on all holes.
I laid out the F-684 gussets on the stock provided. The area at the top and lower left that are marked with hatches need to be removed.
Two sides of the gusset need to be bent a few degrees to allow it to set flush on the engine mount and firewall stiffener.
After very careful checking to ensure sufficient edge distance, I drilled the left gusset to the lower engine mount. I ended up with better than 2d on all holes but the one on the right which ended up at exactly 2d.
The skin can then be peeled back and the firewall stiffener can be match drilled using the holes I predrilled in the gusset.
All of these holes came out great as well with 2d or better all the way around,
I then repeated the process on the right side. This came out nearly as well. The upper left hole came out at slightly under 2d, but that’s still within spec. This finishes up the forward longerons and skin stiffeners.
