On June 22, 2012, at 1330 central daylight time, an experimental-amateur built Yamokoski Glastar was substantially damaged after it made a forced landing to a field in Chicago, Illinois. The private pilot/owner/builder sustained minor injuries. No flight plan was filed for the flight that departed Wittman Regional Airport (OSH), Osh Kosh, Wisconsin, around 1200 and was destined for Southwest Michigan Regional Airport (BEH), Benton Harbor, Michigan. Visual meteorological conditions prevailed for the personal flight conducted under 14 Code of Federal Regulations Part 91. The pilot said he was following the shoreline back to Michigan when he heard and felt two "thumps" from the engine compartment. Approximately 10 seconds later, the pilot heard another thump and the engine RPM began to race. The pilot tried to reduce power by adjusting the throttle, but the engine and propeller were unresponsive. The pilot made a forced landing to a soft field and the airplane nosed-over damaging the right wing strut and the vertical stabilizer. Examination of the automotive engine revealed that the spline-shaft and adapter gears in the reduction gearbox were worn, which caused the gearbox to fail. The pilot reported that the gearbox was not a regularly inspected item for this particular engine/gearbox combination. The gearbox was installed new and had only accrued 247 hours at the time of the accident.