The H shaped plastic coupling next to the expansion tank split in two on my
'84, leaving me dead in the water, so to speak, at the peak of summer here
in Seattle.

This part is obsolete, and only used on the '83-'85 waterboxer.  See Ken
Wilford's chart for location, it's part #29 at
http://www.vanagain.com/19lchc.htm.

Maybe one of you conversion folks out there has one lying around?  I can
send a photo if you think you might.

Boston Bob?  Maybe you have one in a corner someplace.

It's fairly complex, so fabricating one out of copper would be difficult,
but may be the only option.

TIA,

Stuart MacMillan
'84 Westy w/2.1