Complex sentence: Designed under Frank Halford, the engine was very advanced and proved to be difficult to adapt to assembly line efforts, so while the engine was ready for production in 1940, it wasn't until 1944 production versions were considered reliable.
Split into simpler sentences: Designed under Frank Halford, the engine was very advanced and proved to be difficult to adapt to assembly line production. Therefore although the engine was ready by 1940, it was not until 1944 that production versions were considered reliable.