O-ring collapse

["Joe Cantrell" <agiyo@domain.elided>]

Bob McKeown wrote:
In my humble opinion, the roll pins don't help at all. The notion of O-rings
"collapsing" makes no sense to me, since the oil pressure inside the O-ring
is much greater than the (possible) water pressure outside (and there can
only be water pressure present around the O-ring if the head gasket fails).

I agree that the notion of O-rings collapsing makes no sense, but my motor
and several of those I've torn down have indeed had collapsed O-rings.  My
block had no roll pins and 5 of the O-rings were wadded up like pretzels in
the oil passages, and I have pulled heads on rollpin equipped engines where
the O-ring had partially collapsed into the gap in the pin!
I have puzzled over this considerably and was discussing it with another '72
Spider driver just a few days ago.  In none of these cases was there obvious
coolant pressure outside the O-ring; it is almost as though a vacuum forms
in the oil passages.  My current motor has the thin brass rings that sit
flush on the block, but I'm not sure I'd use them again.
What on earth is collapsing those rings into the passages?  Anybody?

Joe

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