The long camshaft for the in-line engines requires the highest precision of such one tenth micron meters but Japan had no sufficient technology, that is, highest-precision machineries to manufacure the part on the massproduction basis. Such important parts were all handmade in those days.
Therefore, IJA, above all, did not pay attention to the liquid-cooled engines at all though they ironically had to pay it later when they understood that they would need it immediately to intercept the super hi-altitude bomber B-29. Completed technology of DB-601 was imported from Germany but everything was too late.
However, there was one chance for Nakajima Aircraft to produce W18 engine(NLH11 Ha-39) because one of their engine designers Hiroshi Takeshima who had already completed his V12 NWE as Ha-15 was so enthusiastic about the development of the liquid-cooled engines.
It was to be produced in their new factory in Ohmiya from 1944.
He had completed NLH11 as Ha-39 by June 1941.
This 1st prototype was very good but broken during the running test because a worker had forgot to pour coolant liquid carelessly. He began to manufacture 2nd prototype but Japan had got into the Pacific War and Nakajima Aircraft had no choice but gave up the liquid-cooled engines.
The new Ohmiya factory was used for the production of double-row 18-cylinder radial engine 'Homare' afterward.
Thanks Aurum for your interest to the history of Nakajima but sorry I have no time to translate the timetable so soon at the moment. I'll try to make it in a few weeks immediately, though.
Please be patient for a while.
Thanks.