The right engine had been replaced as part of a routine maintenance program. During two subsequent flights the engine lost all power. Extensive ground checks failed to reproduce the symptoms and an air test was then carried out. During the test the right engine failed and while various techniques were being employed by the pilot to restore power, the left engine also failed. The pilot was committed to a gear up landing in a paddock and damage was sustained during the ground slide when a mound of dirt was struck. Faulty seals in the right engine fuel shut off valve allowed air to be drawn into the fuel lines, causing the power losses experienced. The left engine failed from fuel exhaustion and when the pilot selected crossfeed from the right tank, air was drawn through the defective valve, preventing further running of the left engine.