Detroit-based General Motors Co. is expanding its partnership with Japanese truckmaker Isuzu Motors by collaborating in the U.S. and marking GM's return into the medium-duty truck market. The companies said yesterday that Isuzu will produce models for GM, based on the Isuzu N-Series trucks, to be distributed by Chevrolet dealers in the U.S., starting next year. GM and Isuzu will also explore the use of GM commercial vehicle components for Isuzu trucks. GM will continue to produce and supply 6-liter V-8 gasoline engines and six-speed transmissions for Isuzu. Isuzu has diesel engines but lacks gas engines. Isuzu and GM, which makes the Cadillac, Buick and Opel cars, have maintained a strategic partnership for more than 40 years.