Zoran and Jeff are correct.The answer, a factory o-ring,  was only 2 miles away at Autosport Imports parts , $3.94 & .37 tax. 

It has squared edges, part number VW # 861 819 297. 

Thanks to all who answered.
Robert
1982 Westfalia

--- On Wed, 12/2/09, Jeff <vw.doka@GMAIL.COM> wrote:

> From: Jeff <vw.doka@GMAIL.COM>
> Subject: Re: Rear heater core valve o-ring source?
> To: vanagon@GERRY.VANAGON.COM
> Date: Wednesday, December 2, 2009, 6:15 AM
> That is correct.� The o-ring is
> round & squared to fit.� Only a Vanagon
> could have a square o-ring...
> 
> Cheers, 
> 
> Jeff
> 
> 
> -----Original Message-----
> From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com]
> On Behalf Of
> Zoran Mladen
> Sent: Tuesday, December 01, 2009 11:58 PM
> To: vanagon@GERRY.VANAGON.COM
> Subject: Re: Rear heater core valve o-ring source?
> 
> I would disagree with that statement.� The last ones I
> bought from VW were
> not typical round o-rings.� They were certainly round in
> shape, but the
> o-ring itself was squared so that it fit perfectly into the
> slot.
> 
> Z
> 
> 
> 
> 
> ________________________________
> From: Dennis Haynes <d23haynes57@HOTMAIL.COM>
> To: vanagon@GERRY.VANAGON.COM
> Sent: Tue, December 1, 2009 9:58:36 PM
> Subject: Re: Rear heater core valve o-ring source?
> 
> Actually the correct ring starts out as an o-ring. Heat,
> compression, and
> antifreeze make it become flat.
> Viton works better than rubber.
> 
> Dennis
> 
> 
> -----Original Message-----
> From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com]
> On Behalf Of
> Zoltan
> Sent: Tuesday, December 01, 2009 8:01 PM
> To: vanagon@GERRY.VANAGON.COM
> Subject: Re: Rear heater core valve o-ring source?
> 
> Basically any O ring that is about the size would do, I
> guess.� The real one
> is flat.� I have reinstalled one before with Right Stuff,
> $15, and it is
> holding well for years.� Right Stuff is a real saver.
> Zoltan
> 
> ----- Original Message -----
> From: "Robert Keezer" <warmerwagen@YAHOO.COM>
> To: <vanagon@GERRY.VANAGON.COM>
> Sent: Tuesday, December 01, 2009 4:45 PM
> Subject: Rear heater core valve o-ring source?
> 
> 
> Hi- is there a vendor who sells a rear heater core valve
> o-ring ?
> My heater core is fine, just that it leaks at the o-ring.
> The o-ring is old.
> What are the tricks to fix? A $4.00 tube of silicone
> sealant or a $2.00
> o-ring? Or a $3.00 o-ring? Or a .99 cent o-ring with 10.00
> S&H? Just kidding
> on the last one.
> It looks like a special o-ring, or is this just the way it
> looks, with a
> groove on one side, after many years?
> 
> thanks
> Robert
> 1982 Westfalia
>