On April 4, 2013, at 0840 eastern daylight time, a Cessna 152, N6141Q, was substantially damaged during a forced landing following a partial loss of engine power near Miami, Florida. The certificated flight instructor and the certificated student pilot were not injured. Visual meteorological conditions prevailed, and no flight plan was filed for the local flight, which originated from Kendall-Tamiami Executive Airport (TMB), Miami, Florida, at 0745. The instructional flight was conducted under the provisions of Title 14 Code of Federal Regulations Part 91. At the conclusion of an uneventful training flight, the flight instructor asked the student pilot to demonstrate a steep turn prior to returning to TMB. While in the turn, the engine experienced a partial loss of power, and the propeller continued to “windmill.” The flight instructor then took control of the airplane and returned to straight-and-level flight before attempting to restore power to the engine. After determining that engine power could not be restored, and that the airplane would be unable to reach TMB from their current position, the flight instructor elected to perform a forced landing to a road. During the landing, the left main landing gear caught on a grassy area next to the runway, yawing the nose left. The airplane subsequently struck a bush, resulting in substantial damage to the right wing. The engine was examined following the accident under the supervision of a Federal Aviation Administration inspector. Internal examination of the number 3 cylinder showed that the exhaust valve head had fractured and was separated from the valve stem. The valve head was recovered from the engine cylinder, and showed evidence of post-separation damage and contact with the interior of the cylinder and piston dome. No other mechanical deficiencies of the engine were identified during the examination. The accident airplane was manufactured in 1981 and was equipped with a Lycoming O-235-L2C engine. Review of maintenance logs provided by the operator revealed that the engine was installed in the accident airplane following a field overhaul in June 1985. At the time of the overhaul, the engine had accumulated 1,994 total hours of operation. In December 1985, after the engine had accumulated 299 flight hours since the most recent overhaul, inspection of the number 3 cylinder revealed a broken rocker shaft boss. The remedy to the discrepancy included the installation of “serviceable valves.” Between 1985 and the date of the accident, no other replacement of the number 3 cylinder or its valves was noted. No other documents accompanied the log entry, and the prior service history of the installed “serviceable” valves could not be determined. Based on the maintenance records provided, the engine’s maintenance history between 1985 and 2011 could not be determined; however review of an entry dated October 2011 noted that the engine had accumulated 1,876 total hours since its most recent overhaul. The airplane’s most recent annual inspection was completed on April 14, 2012. At that time the engine had accumulated 5,702 total hours, and 1,929 hours since overhaul. The most recent 100 hour inspection was completed on March 26, 2013, and at that time the engine had accumulated 2,408 hours of operation since the most recent overhaul. At the time of the accident, the engine had accumulated 2,495 hours since the most recent overhaul, and 6,269 hours since its manufacture. According to Lycoming Service Instruction 1009AS, revised May 25, 2006, the recommended overhaul interval for the O-235 engine was 2,400 hours or 12 calendar years.