Much depended on what kind of changes had to be made. Some parts were easier to come by then others, especially high tech parts were hard to obtain on time. Also, when the bombardments on building yards intensified it became progressively more difficult to get the job done. A complete overhaul which normally takes a few days would be stretched to weeks because of the bombing raids. Also, larger parts had to come from Germany by rail and often these wagons with parts were put on a sidetrack until the main tracks were repaired again.
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