I feel your pain! It's fairly common for -4's to suffer wrinkling in the lower corners, usually as the result of a hard landing load or continuous operation off of rough surfaces. The aluminum fuselage structure flexes under load (as it's designed to do), but the hard stainless firewall suffers stress wrinkles or cracks. As long as there is no permanent fuselage primary structure or landing gear leg deformation, the firewall itself is non-structural. If the stainless has any holes or cracks, they should be smoothed and/or stop-drilled, as appropriate and filled with high temp silicone to preserve firewall integrity.
Similar to your situation, I also watched my brand new replacement lower RV-4 firewall crack in the lower corner within a few landings because I didn't "pre-stress" the lower firewall thru-bolts...i.e., before drilling the replacement firewall rivet holes and riveting, use a nut, bolt and fender washers to properly compress the stainless in the corners to mimic the compression imparted by the engine mount. So that would be my primary suggestion in addition to adding the RV-8 style gussets as suggested by Van's!
Likely you are already aware, but be sure to check the engine mount for any cracking as well, since the same stress that induces the firewall wrinkling causes stress at the welds in the gear leg sockets. Engine mount fatigue cracking is not uncommon (mine cracked at about 1100 hours). If your mount is finished in black powder-coat, visual inspection can be difficult, and may require you to remove the powder coat in suspect areas to make a determination. If the engine mount is dark, sometimes the presence of a small amount of grease at the weld can provide a visual cue that the weld may be cracked. If the weld cracks, it's possible for some of the grease to be forced out of the gear socket through the failed weld as the area flexes during taxi, takeoff or landing. Given the option, a painted mount that uses dark primer and light finish color can simplify visual inspection.
Last edited by Vac : 11-06-2017 at 11:19 AM.