WD403 gusset
Here's a fuzzy photo of my WD403 gusset.
http://picasaweb.google.com/lh/photo/-khCFvnPR8ESihhRNIz8mg?feat=directlink
As shown, they're pretty simple to make from .063 alclad. The layout goes as follows:
The height was determined by SWAG. The front edge was set 1/16" or so back from the firewall, and the back edge is flush with the back of the side face of WD403. The bottom edge, not visible in the photo, is cut flush with the bottom of the diagonal longeron. Finally, a notch is cut out of the bottom front corner for clearance around WD403. Use a generous radius on the inside corner of this notch (i.e., pre-drill the corner with a 1/4" drill).
I made a bend line 3/4" up from the bottom edge, and bent the flange 5 degrees or so to match the angle of the mating face of WD403. The bend is right when you can put the piece flush against WD403 with the front edge of the gusset resting nicely against the vertical brace on the firewall. At this point, you'll notice another slight bend is necessary for a good fit against the vertical firewall brace. Mark a line 3/4" back from the front edge and make the bend accordingly. This one is only 2-3 degrees.
I'd have liked to put a flange along the diagonal edge for rigidity, as it would have made the part significantly more resistant to buckling under the types of loads we see on a hard landing. However, I just don't have good equipment for putting large bends in thick 2024 sheet. I think the gusset as-is is pretty close to the Van's design in the later models, and it greatly strengthens the original design by eliminating a load concentration at the vertical firewall brace just above WD403.
My guess is that this design will eliminate the common firewall cracking around the lower engine mount bolts. With thousands more load cycles, cracks might eventually form in the firewall about halfway up the vertical firewall braces, either at the brace rivets or in the corner next to the flange that joins to the side skin and piano hinge.
Tailwinds,
M