The biggest hole on the ship is about 28” in diameter. Some things that need to be unshipped are somewhat bigger than that.
On the original Polaris missile subs, it was even smaller - - 22" as I recall. The boats had three inertial navigation systems, which were too large to be loaded on board fully assembled. The gyros and accelerometers had to be aligned after installation to an accuracy of an arc-second or so. This was done by lapping the interface surfaces and checking the alignment with manually operated optical instruments. All this was before lasers were available for such precision work; it all depended on technicians working with their bare hands. (Accuracy of the ballistic missles was totally dependent on how well they did the alignment job).