Event
A pilot en route to a field for aerial application observed an oil mist on the windshield. Shortly thereafter, the engine experienced a progressive loss of power, prompting the pilot to execute a forced landing in a pasture. During the landing roll, the aircraft struck a hole in the ground and nosed over.
Inspection Findings
Post-accident examination of the engine revealed that the exhaust rocker box on the #1 cylinder had cracked and separated at the attachment points for the oil lines. The cylinders were Pratt & Whitney R-1340-AN1 models. Two Pratt & Whitney service bulletins concerning these cylinders had not been complied with. Service Bulletin (SB) 1720 recommended an inspection of cylinder heads during overhaul. SB 1743 called for replacement of all cylinders with improved cylinders featuring a 5/8 inch reinforcement web on the exhaust rocker box, whereas the old cylinders had a 1/2 inch web.
Maintenance History
The aircraft had flown 360 hours since the last major overhaul.