Originally Posted by Dayton Murdock
What is flexing to cause this to happen. Until that is resolved this deformation will occur again.
There was a design change of the steel weldment in the four corners of the firewall longeron attachment located in the cockpit. (Wd-402L, Wd-402R, Wd-403L, and Wd-403R) It deals with early RV-4s. Later kits were shipped with an improved part. I thought there was a SB for it but cannot find it. The change was published in the early RVators. If you really want to know more, I will dig through my early paper copies of the RVator and email it to you.
Google search found this
"Engine Mount Bracket Reinforcement"
"Several RV-4 pilots have reported cracking failure of the engine mount brackets (WD-402 & 403) in the forward fuselage. Usually this has resulted from continued operation from rough airstrips or one or more extremely hard landings. Regardless, these brackets seem to be the structural Achilles Heel of the RV-4 airframe."
"Some time ago, we altered the bracket design by increasing the thickness of the attachment arms from .050 to .065". However, many RV-4s now flying have the .050 brackets. We have developed a couple of relatively simple mods which can be added during or after construction."