Someone last year posed the same question and here was my reply:

Water Tank Caps:
80-84  VW # 253 067 285 (Size unknown but different from late style, I think
larger)
85-91  VW # 255 070 802B (Size 35x4)

Waste Water Cap Assembly (VW calls it an "Evacuating Valve")
All Years  VW # 255 070 807
Note:  As posted by Mark earlier, the cap by itself is the same threads as a
common garden hose, easily found at any Lowes or Home Despot.

Jim Thompson
84 Westfalia 2.1 "Ole Putt"
oldvolkshome@earthlink.net
jim@karmannghia.com
http://www.oldvolkshome.com
***********************
----- Original Message -----
From: "Mike Miller" <mwmiller@CWNET.COM>
To: <vanagon@GERRY.VANAGON.COM>
Sent: Wednesday, February 09, 2005 10:55 AM
Subject: Water drain cap

> Volks,
>
> Anyone know if the water tank cap and the sink drain cap are the same on
an
> '89.  They aren't on my 85, sink drain is much smaller than the water tank
> drain.
>
> Also are the 89 caps the same size as the 85 water tank drain cap?
>
> Mike