Was this a Chinese cap?� Have seen a bunch of them...all crap...exact same issue.� The cap stays on ok until it heats up and then pops off.
�
Beware of Chinese parts...never heard that one before!
 

________________________________
 From: WILLIAM WITZ <pbwitz@SBCGLOBAL.NET>
To: vanagon@GERRY.VANAGON.COM 
Sent: Thursday, September 26, 2013 7:00 PM
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.