On April 10, 2012, about 1315 Pacific daylight time, a Willerton Vans RV-6A, N6000Z, sustained substantial damage to the firewall during a precautionary landing at Blue Ribbon Airport (WN29), Sequim, Washington. The certified flight instructor (CFI) and private pilot were not injured. The airplane was registered to and operated by the private pilot. The local instructional flight was conducted under the provisions of 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed, and no flight plan had been filed. The flight originated from Sequim Valley Airport (W28), Sequim, Washington at 1300. The private pilot reported that while climbing through about 4,000 feet, both the CFI and he observed fuel leaking from the right wing fuel cap. They initiated a precautionary landing to WN29. After a normal approach, the airplane struck the runway hard causing the propeller to strike the ground. Subsequently, the airplane’s firewall was substantially damaged. The pilot reported no mechanical malfunctions or failures that would have precluded normal operations. Post-accident examination of the aircraft was conducted by a Federal Aviation Administration (FAA) inspector. The stopper type fuel cap was found locked; however, the fuel cap was loose and not properly secured within the filler port.