Prior to the accident flight of the experimental homebuilt, the spark plug inserts in the engine cylinders were changed to steel ones. The pilot said that during the flight he heard a loud noise emanate from the engine followed by a partial loss of power. The pilot said he was able to maintain an altitude of 1,000 ft AGL and returned to the airport. On final approach, the pilot did not get a landing clearance 'due to other voice traffic' on the radio and initiated a go around. The pilot further said that he could only maintain 300 ft AGL on the second pattern. A witness said the aircraft made a normal approach then 'bounced up at a high angle on touchdown, then drop on the nosewheel.' The pilot reported that examination of the engine revealed 'a sliver of steel from the spark plug insert had lodged in the cylinder head causing a blow torch effect which burned through the cylinder head and fin.'