The pilot reported that after a normal takeoff and landing gear retraction, he began to level the airplane at 4,500 ft above ground level when the left engine began to run rough. The pilot and pilot-rated passenger then reduced power on the left engine until the roughness abated and agreed that they should return to the departure airport. While on the downwind leg of the traffic pattern, the pilot placed the landing gear handle in the down position, but the landing gear down indication light did not illuminate. The pilot and pilot-rated passenger attempted to troubleshoot the issue before the pilot ultimately decided to attempt a manual extension of the landing gear. The pilot-rated passenger then continued manual extension and was able to turn the crank approximately 50 times before it would no longer turn, and he believed it was “against the stops.” The pilot still did not observe an indication that the landing gear were extended, but decided that they needed to land as a result of the difficulty in maintaining altitude and airspeed. After touchdown on the main landing gear, the nose of the airplane settled onto the runway and the airplane skidded to a stop. A bulkhead in the airplane’s fuselage structure was substantially damaged during the landing. Following the accident, maintenance personnel examined the airplane and reported that their initial attempts to extend the landing gear normally were unsuccessful. After they “tapped the [landing gear] relay with a hammer,” the landing gear extended and retracted normally during several test attempts. The landing gear were also successfully extended manually after the manual extension crank was turned about 10 additional times beyond where it was found following the accident. Additionally, examination of the left engine identified two oil-fouled spark plugs as likely contributors to the engine roughness reported during the accident flight.