Seven examples of the proposed Reggiane Re.2004 were ordered. However, engine development was proving difficult; the first Zeta R.C.45 was run on 28 February 1941,rated at , but would only develop , as well as having many other issues. Resolving these took time and when the more powerful Zeta RC24/60 was run in May 1943, it still failed its type test. Reggiane tested a mock-up in a wind tunnel and mounted it in the nose of a Savoia-Marchetti SM.79 but cooling problems continued to hold development back. It was not until 14 August 1943 that the first engine took to the air in a Caproni Vizzola F.6Z, nearly two years after the Daimler-Benz-powered F.6M. Further development ceased following the armistice on 8 September 1943. By this time, Reggiane was still waiting for their first engine. As a consequence, the company had already started developing their own engine, the 18 cylinder Reggiane Re 103. The sole F.6Z remained the only aircraft flown with the Zeta.