On February 18, 2011, at 0958 mountain standard time, a Rearwin 8135, NC25545, made an off airport forced landing near Casa Grande, Arizona. The pilot/owner was operating the airplane under the provisions of 14 Code of Federal Regulations (CFR) Part 91. The commercial pilot was not injured; the airplane sustained substantial damage to the fuselage and wings. The cross-country personal flight departed Eloy, Arizona, about 10 minutes earlier, with a planned destination of Chandler, Arizona. Visual meteorological conditions prevailed, and no flight plan had been filed. The pilot stated that he was in cruise flight at 2,500 feet, when the engine temporarily cut out, so he started looking for a place to land. The engine cut out again; he applied carburetor heat and manipulated the mixture to no effect. He made a high approach to an open field due to power lines. He said that this made the airplane too high and fast for the space available for landing. The airplane landed in a rough, uneven field, and collided with a berm at the end of the field. The left main landing gear buckled, the airplane went up on its nose; the airframe sustained substantial damage. A Federal Aviation Administration (FAA) inspector oversaw an engine examination at the facilities of Air Response, Mesa, Arizona, and his report is in the public docket. The overall appearance of the engine looked good, and the only visible damage at the onset of the inspection was damage to the left side of the exhaust tail pipe. Mechanics removed the forward spark plugs from each of the cylinders, and using a crow bar against the propeller mounting bolts, were able to turn the engine 720 degrees smoothly without any difficulty. Each of the spark plugs was examined. Plugs from cylinders number three, four, and five showed signs of oil in their respective cylinders, which they indicated was not abnormal for a radial engine. Cylinder number four did exhibit copious amounts of oil inside the cylinder when the engine was rotated; again they observed that this would not be abnormal for a radial engine’s lower cylinders. However, they pointed out that the oil in the cylinder was enough to create a hydraulic lock if it were not removed before attempting to start the engine. The FAA inspector asked the owner/pilot if he rotated or pulled the engine through before starting the engine after prolonged periods of inactivity. The pilot stated he had not, and did not feel that it was needed, as he has never had a problem with hydraulic lock in the past. Once the mechanics drained the oil from the number four cylinder, they performed a compression check of the engine. The results were as follows; 1-66/80, 2-20/80, 3-70/80, 4-80/80, 5-78/80 6-80/80 7-60/80. They indicated that even though cylinder number two was low, they did not believe that it would have caused the type of loss of power and the roughness the owner described. The FAA inspector asked the mechanics to check the valve train to be sure nothing was wrong with the system. They found that the cylinder number four exhaust valve rocker box was loose. When they removed the baffling to inspect the rocker box further, the box was found not to be attached to the head of the cylinder. Three of the studs where broken, and the other three studs threads had pulled out of the head. One of the three studs, which had pulled out, showed evidence of being loose in the head for some time as the stud was rounded off. The adjacent stud was rounded off, but not as much, and the final stud had no signs of rounding off. Three out of the six studs were broken off in the cylinder head and rocker box. The first one on the right side top near the grease zerk fitting of the rocker box had a remnant of a thread, but the surface of the stud showed beach marks and staining as though it had been broken for some time. The second stud on the left side of the zerk fitting was broken off below the surface of the rocker box. Because of oil and dirt, it was difficult to make out any distinguishing marks other than one 45-degree lip and thread. The last of the broken studs had a 45-degree lip through 1/2 the stud, but the grain pattern was consistent over the entire top of the stud with one exception in the thread line. The overall consensus of the mechanics involved in the inspection, and the FAA inspector, was that the breaking of the studs and the pulling of the threads could cause an abrupt loss in engine power because the exhaust valve for that cylinder would no longer open and close properly. Should an exhaust valve fail to open after that cylinder has fired, the combustion flame would propagate through the induction system, and rob the remaining cylinders of a fuel air charge. This would repeat continually until the engine quit, or the valve opened.