A Cessna 210M lost engine power and collided with a pole after a forced landing on a city street. The engine crankcase was fractured and the #5 piston connecting rod had separated from the engine and was found along the wreckage path. Examination of the number 5 cylinder wall disclosed evidence of the area below the normal piston stroke being scraped by the piston rings. According to the metallurgic report, the piston's connecting rod failed due to a fatigue crack. The connecting rod was bent some time after initial engine operation. The bending that occurred to the No. 5 connecting rod is characteristic of a hyd lock condition where the piston is forced against incompressible fluids between the piston and cylinder head. No indication of metallurgical defect was observed in the connecting rod. Hardness readings on the connecting rod, which indicates the strength, conformed to material specifications. The maintenance records note that a major overhaul was accomplished 207.8 hours before the accident.