It's just an o-ring, take the circlip out and get that plug out. Goto a parts store and get a o-ring the same size as the old one. I did the same thing with my M50.
91 318is - M50NV (gone)
02 WRB Subaru WRX EJ207 forged, 320 AWHP stock turbo.
85 - 325
02 325i Sedan