So I had a quick look at my E53 which I installed the (supposedly) same expansion tank into a while back and the clip is very tight so the slot is obviously there so I would say that maybe someone did a rebox and sent back a cheaper tank. I have had "the switch" happen with Felpro gaskets from RockAuto once before, but they have the tank on at such a low price from what it was, it's probably more likely that it's just a different tank being put into the box. ;( The tank is doing fine without the clip holding and I'm fairly sure it's not needed and maybe even the slot left off to make installing easier but I'd feel better if someone else had some first hand knowledge with this. I guess maybe people returning the tank because the slot wasn't there might even be why they are selling so cheap??

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