Yes, the discs have a soft metal center that will melt
away if the surface it's glued to overheats, like on a
cylinder head.  Imagine a life savers candy with a
zots candy center.  If the center's gone you know it's
been previously sucked.  :^)

On a head I almost needed for my Vanagon the disc was
glued to a blank in place of one of the head coolant
drains on the bottom.  Thankfully I didn't need it and
was able to return it unused to my favorite Seattle
area VW machine shop.

A good machinist uses them to ensure a return wasn't
overheated.  Especially on alloy heads it's very
important.

Gregory Smith
'84 Vanagon
Olympia Wa

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