Dats-hot's on the mark.

Don't forget, Nissan was a partner of Austin, and they made -- not copies -- but actual Austins under license. The agreement called for Nissan to make most of the components locally within 4 years. So the Austin engines made in Japan actually were an improvement. For example, Nissan could only get good forged steel, so the cranks were better.

The lessons learned from Austin were of course applied to Datsun development. The E-series engine (predecessor to J-series) were an improvement over the Austin design. Higher compression, shorter stroke, better airflow.

The A-series was the all-new engine, sharing nothing with the Austin engine. It was based on an all-new lightweight casting.