Why wartime Britain still needed American Merlin production

Would overseas engine production remove the need for American Merlin manufacture?

No; Britain needed every available engine and also imported American machine tools to expand Merlin factories at home before US engine lines reached full output.

British factories diverted some machine-tool production toward weapons, creating a need to import plant for additional engine lines. The example makes logistics tangible: wartime output rested on tooling, spares, and redundant factories, so an overseas line complemented rather than replaced American and British production.