That style cap is basically identical to radiator caps used on American cars
for years, back before plastic tank radiators.
If you take it to a FLAPS ......if they're any good at all, they can match
it nicely.
The factors are ........how it grabs around the outside of the 'radiator' or
expansion bottle neck, how far down it reaches to seal on that 'shelf' sort
of inside the neck or opening; it needs the return valve, and the pressure
rating is 14 psi , or right in there, very close to 14 psi.

  Could be you already tried this, and the cap you got didn't work and fit
quite exactly right, but it could be worth seeing what a FLAPS has -  that
style and pressure cap is extremely common. Unless you can get the exactly
correct German one easily of course.

----- Original Message -----
From: "Alistair Bell" <albell@SHAW.CA>
To: <vanagon@GERRY.VANAGON.COM>
Sent: Sunday, August 23, 2009 4:36 PM
Subject: Re: 82 diesel expansion tank cap

> there is a BMW cross ref. cap that works, I used it on my '82.
> Unfortunately it does not have any codes or makers marks, just "zu"
> and "auf" arrows and a warning in german about cap removal ( I think).
>
> and the number "100"
>
>
> maybe this helps? :)
>
> Alistair
>
>
> On 23-Aug-09, at 1:29 PM, Alan Felder wrote:
>
> Ok, I've been surfing the net for several hours now for a replacement
> expansion tank cap (metal) for my 82 diesel westy.  None of the usual
> suspects seem to have them, nor the scads of other sources.  Anyone
> have a
> clue where these might be found?  They are NLA.
>
> --
> Alan Felder
> Austin TX
> 82 Diesel Westy