My 2016 Clubman cooper S went in for a pre winter health check and engine oil service, mileage at the time was 12,675 during the health check inspection gearbox oil was seen to be leaking from the gearbox case end plate, it is a rubber grommet bung of 37mm ø this rubber grommet bung was now no longer supplied by MINI and was impossible to source anywhere, this resulted in an eye watering £5,038 factory new gearbox and clutch replacement, the clutch was not needed but I insisted a new clutch be fitted,

so a word of warning if your gearbox starts to leak from this rubber grommet bung, this part is no longer supplied, and unless a DIY repair could be done, then it’s time to find a decent used item.

 

Had the same leak crop up on my wife's F56 Cooper - thankfully the car was under CPO warranty at the time, and they called for a full trans replacement. Seemed unnecessary at the time, but I wasn't going to argue.

Seems like the part may be available though?

https://www.realoem.com/bmw/enUS/sho...diagId=23_1223 (#10 on diagram, p/n 23008484418)