Originally Posted by
tdracer
I never worked the -535C, but my recollection is that the engine/aircraft interface didn't change between the 535C and the 535-E4 (just maybe some pin selections for the EICAS and FMC to account for the different engine limits and fuel burn).
If my memory is correct, it's not impossible that a particular aircraft might have switched between -535C and -535-E4 more than once. Aircraft comes in for heavy maintenance, they drop the engines and send them off to overhaul, then install whatever type they have a pair of handy (I'm pretty sure intermix wasn't allowed so both engines would need to be the same type).
The pylons were structurally different for the two engine types. At one time BA a had a C powered aircraft with a cracked pylon and it was hoped that a replacement pylon could be bought surplus to AA their C to E mod programme but none could be found, presumed scrapped, so a costly and lengthy repair to the original pylon had to be carried out.