Outer Seal - Yes or No

[dspoljaric]

For use with the original metal expansion tank, I have conflicting information on which cap is correct - with or without an outer seal?

One source states the following:

Older metal cooling system caps manufactured by German OEM suppliers like Blau and Reutter for vintage BMWs (such as the E3, E9, E10/2002, and early E21/E30 models) feature a distinct internal metal and rubber configuration:
• Upper Plate: The upper sealing surface relies on a solid brass, copper, or stainless steel metal disc that seats directly against the upper lip of the radiator neck. It does not utilize an attached upper rubber gasket. Instead, it acts as a mechanical metal-to-metal atmospheric seal to guide overflow fluid safely down the overflow tube.
• Lower Seal: The critical pressure-holding element is located at the bottom of the spring-loaded plunger. This contains a thick lower rubber seal that presses firmly into the lower seat of the radiator neck to maintain the required 1.0 bar (approximately 14.5 to 15 PSI) cooling system pressure.

Another source states that you need both the inner and outer seal.

Thoughts?

 

[HB Chris]

Interesting, I’ve never taken notice of the cap seal. First pic could be original cap from my `70 2800, it has both seals and no name. Next pics are on my e9 and is newer, has BMW logo. To me the outer seal is redundant as the inner one seals the opening.

 

[dspoljaric]

Thanks Chris

 

[HB Chris]

I can see now why the outer seal is needed, if the main seal allows pressure out then this keeps the overflow in the outlet tube and not around the cap.

 

[Markos]

No idea on correctness but I have both also. The BMW one doesn’t have the outer rubber. These have been media blasted so no zinc.

 

[HB Chris]

The one on the right has a larger seal and a plastic center, the locking tabs also look too large to be BMW.

 

[Markos]

The one on the right has a larger seal and a plastic center, the locking tabs also look too large to be BMW.

Center is rubber over metal. Agreed that the tabs look different but it fits on the tank and came with it, so it was presumably in use. I don’t intend to use either of them.