That's what I was going to say, mine is a 2004 and obviously improved from yours. I measured everything to do with the O-ring portion and it's all good. I measured about 0.2mm clearance(although there does seem to be a taper) and the o-ring is 2mm thick. I'm curious though about what's in that section? is there a valve or is it just a flow divider for the 2 syphon jets?

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1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)