unless gear box seal is leaking leave it alone as with collar in place can be tricky to get out and in without damaging collar or fitting seal square, intermediate shaft will tap out or enough to change oring good idea to put some silicone around too box side of oring ,on td5 the 30mm spanner size nut can be locked up tight ,but 300 tdi and earlier cant be done quite so easily as the brgs have collapsable spacer between, while td5 has solid selectable spacer, put vaseline or such on inside of seal if tapping inti place to keep garter seal in place, never seal between boxes as are designed if there is a leak for it to run out between them there is a little channel on gearbox casing for this and not transfer between them