Seals on Expansion Tank

[Duncraig]

I've searched but can't find an answer.
I'm new to Z3 ownership and I traced a water leak to the expansion tank seal so I ordered a new tank as it needed a new pressure cap as well. The new tank and the old one have only one large 'O' ring fitted although there appears to be a place for a second ring in another groove. Some of the photos of the tanks show them with two seals. Why is this?

 

[colb]

http://www.realoem.com/bmw/enUS/showparts?id=CM12-EUR-12-1999-Z3-BMW-Z3_19&diagId=17_0089
Link to 1.9 car shows the parts fitted.
If you put the last seven digits of your Vin number in on the home page it will display parts applicable to your actual car.

 

[Duncraig]

http://www.realoem.com/bmw/enUS/showparts?id=CM12-EUR-12-1999-Z3-BMW-Z3_19&diagId=17_0089
Link to 1.9 car shows the parts fitted.
If you put the last seven digits of your Vin number in on the home page it will display parts applicable to your actual car.

Thanks, I've checked there and I've got the correct tank for my car, I just wondered why some tanks have two seals and others only one when there's grooves for two seals on the tank.

 

[MalcQV]

I've just replaced mine (well a couple of months ago) and it has two seals as did the old one.

 

[Duncraig]

Yes, my old has one seal, the new has one, BMW parts show one but some seller's tanks have two

 

[Aceman]

It is strange but BMW do have a habit of changing the spec on some parts without explanation so it could be that you have obtained old stock and new produced tanks now have double seals or visa-versa, lots of parts have changed over the years probably from BMW sourcing from different manufacturing houses. Don't think anyone will have a proper explanation other than BMW themselves and even then it can be like extracting blood from a stone.

 

[Duncraig]

It is strange but BMW do have a habit of changing the spec on some parts without explanation so it could be that you have obtained old stock and new produced tanks now have double seals or visa-versa, lots of parts have changed over the years probably from BMW sourcing from different manufacturing houses. Don't think anyone will have a proper explanation other than BMW themselves and even then it can be like extracting blood from a stone.

Thanks for the explanation, managing to learn about my car thanks to the forum. Need to ask which oil for the diff but I'll start a fresh post for that.