URGENT advise needed

[e60]

Advise needed!
I own a E60 with a N53B30A, LCI E-shift 6HP21. My automatic transmission has started slipping and ive decided to repalce it with a low mileage/Km one. The question i have: i found another 6HP21 ehich came from another car with the same engine and same LCI E-shifter, but how do i know itll fit my car eintout any modifications needed like “plug n play”. Ive alot of people tell me these gearboxes are compatible from 3 series to 5 series if tbey got the same engine, but some “expert” told me it wont fit beciase thr entire shell is a diffrent size from a 5 serir in comperison of a 3 series and so wont work. I need someone to advise me on how it works and what is and is not compatible! Thanks in advance!!!! ( check pictures for my numbers on gearbox)

[Ralph III]

Exactly what 5 series is this, the mileage, and model year? 95% of the time a slipping transmission is caused by an issue with the mechatronics unit. It is the mechanical/electronic module that controls the transmission. The seals and/or solenoids will fail and that will lead to a slipping transmission. There are many threads on this forum and others documenting the service of the transmission/mechatronics unit.

It's a little bit involved but this can easily be serviced by any DIY with a little bit of patience. I think I could perform the service on our vehicles in less than 5 hrs now. Anyhow, it's extremely rare that the transmission would need to be replaced. In fact, I only know of one case in which a highly qualified DIY/Mechanic opted to replace his transmission. It was a very unique situation for him though, as I recall.

Ralph