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An ultrasonic soldering process was developed for manufacture of a compact and lightweight water pump case for sport-utility motorcycles. Most standard water pump cases consist of an aluminum pump case and a steel pipe brazed to a steel flange. Conventional joint method of an aluminum pump case to a steel pipe is by bolting. Our modified ultrasonic soldering succeeded in joining an aluminum pump case and a steel pipe without any flange or bolt. Also by adopting an oval joint shape, the water pump case became more compact and has been produced with elimination of 4 parts. Its weight was lessened by approximately 16% from the previous one. It has been considered difficult to join steel and aluminum because of the formation of fragile intermetallic compounds. We prevented, however, the formation of intermetallic compounds by ultrasonic soldering. We verified that water pump cases joined to a steel pipe by ultrasonic soldering have excellent joint strength and high reliability.