Wikipedia puts the CF6 from a 747-400 at 4.27m long, 2.69m diameter. An RB-211 looks to be a little narrower and longer. Cargolux puts the biggest rectangle that can fit through the main cargo door at 3.05m x 3.10m. So yeah you could probably fit an engine in a nacelle and fixture through the main deck door on a freighter.

However your big limitation will be height at either 2.44m or 3.00m depending on where on the main deck you're checking. The lower deck (which is all you'd have available on a passenger 747) has a much lower height limit (an LD-1 has a height limit of 1.62 m).

An engine's just not going to fit inside unless you're talking about something designed for oversize cargo like the Dreamlifter (747) or Beluga (A300) or Beluga XL (A330). Oversized cargo is also the main use case for the An-124/225.

https://www.cargolux.com/fleet-equipment/aircraft/747-400f-s...

> An engine's just not going to fit inside unless you're talking about something designed for oversize cargo like the Dreamlifter (747)

This is in direct conflict with literally the paragraph you posted before it. You could definitely carry the RB-211 on a standard cargo version of the 747, as you yourself stated previously. It could only fit on the main deck, but the standard 747 cargo (aka not needing the guppy) definitely could do it.

Yeah there's about 0.7 m more height than a CF6. That seems like potentially not enough room for a proper fixture. In any case it's not going to fit inside a passenger 747.