The diameter is 1150 mm. The 12 weighs 530 kg dry and the 21 weighs 590 kg dry. The bore is 130mm and stroke 150 mm. That's all I can find at the moment as I'm busy this time of year, but try this link and I'll bet something of interest is in the book. It's to a pdf I have The Nakajima Aircraft Engines book.
The diameter is 1150 mm. The 12 weighs 530 kg dry and the 21 weighs 590 kg dry. The bore is 130mm and stroke 150 mm. That's all I can find at the moment as I'm busy this time of year, but try this link and I'll bet something of interest is in the book. It's to a pdf I have The Nakajima Aircraft Engines book.