Replacing submarine diesels can require cutting open the pressure hull

Why were submarines with foreign diesels not refitted with Admiralty engines when spare parts ran short?

The engines were difficult to remove without cutting open and resealing the hull, making replacement slower and riskier than building anew.

Boats with German MAN or Swiss Sulzer engines could not be standardized casually on Admiralty machinery when spares became scarce. Cutting the hull to extract the engines created cost, delay, and a permanent structural concern, so continued use or new construction was usually easier.