The Nakajima model designation for this engine was NAK while it was an experimental project, in service it was known as the NK7, and known as the Ha-103 by the Army and "Mamori" or "Mamoru" by the Navy.

Competing against the successful Mitsubishi Kasei engine, the Mamoru proved to be unreliable and was rarely able to operate near its design power.

Designs based on the Mamoru were forced to use other engines, typically the less powerful Mitsubishi Kasei or the Nakajima Homare.

In an attempt to salvage the project, two additional airframes were fitted with 1,530 hp Mitsubishi MK4B 12 Kasei engines and redesignated G5N2s.

With the delay of 2 years, by 1943 the engine had improved to the point where serial production was allowed to start, but after only 133 B6N1s had been delivered the Navy ordered the switch to the 1,850 hp (1380 kW) Mitsubishi MK4T Kasei 25.