Sounds to me the like the piece that the cap screws or locks to, is worn out
and rusted.
It's secured by 3 phillips head screws, that screw into a very solid piece
metal on the inside of the fender.
 A good used one of those - the piece the cap screws too ( if you can get
the 3 screws out nicely ) sounds like what you need. Either that or a whole
early style metal filler neck with all the small parts to go with it. 
 I might have a very good, not rusted, west coast set up for you, if you are
interested.  
Scott
www.turbovans.com 

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of
Sam Conant
Sent: Wednesday, September 19, 2007 3:52 PM
To: vanagon@GERRY.VANAGON.COM
Subject: Gas Filler Tube Info Inquiry

My 84 Westy's gas tank filler tube entry is finally about to give up the
ghost as years of rusting have finally taken the toll.  The edging which
"locks" the gas cap in place, are almost rusted through, and I expect will
break off in the next 1-3 fill-ups.
Does anyone on the list know if anyone fabricates and sells liners (with
lock-related edging) which can be slipped into the filler tube.  This would
surely save me some precious $$$ if available.
Tks
SamC
Colchester, Vermont