The threads should not seal anything. The seal is at the top of the opening and the face gasket on the cap.

Dennis,
From my phone.
________________________________
From: WILLIAM WITZ<mailto:pbwitz@SBCGLOBAL.NET>
Sent: ‎9/‎26/‎2013 8:00 PM
To: vanagon@GERRY.VANAGON.COM<mailto:vanagon@GERRY.VANAGON.COM>
Subject: Expansion  Tank Defect

Hi  all, just a heads up to let you know that I had to wrap a bunch of Teflon tape on my new Expansion tank threads in order to get the Cap to seal on the tank .  The cap was new as well . Very poor fit on the threads. I ran it on the driveway until the Radiator fan came on. The threads then sealed as they should. We got our Westy back after a manual trans rebuild, it is now a joy to drive.
Bill Witz 84 Westy Tiico P.S. we are going bird watching on Friday although it may be to early.