I was enroute from Lyneham to Akrotiri in a Britannia when number two engine tried to shed it's cowling near Rome one night. I diverted to Luqa where the engineers decided to replace the whole cowling with one from the spare Proteus stored in the MU at Safi. It took seven hours of filing, sawing and hammering to get the damn thing to fit. Sometimes, they told me, it was quicker to change the complete power plant due to manufacturing variations.