I have read and heard that the Packard built engines, built to mass production automobile standards, was easier to assemble and more reliable then the Rolls version. Apparantly the tolerances were much more precise so that it just went together like a jig saw puzzle rather than hours with a file.
I found this working on American and British car engines in a past life. The American components came off the shelf and went straight in; with a British engine you had to look for over/under tolerance stamps on the block to check the fitting.