I think it's intended to keep coolant away from the dissimilar metals that
are in direct contact there (steel and aluminum alloy).  When these rigs
were built antifreeze had short lived corrosion inhibitors that formed
acids, which is why two  years was the maximum on them.  Today we have long
life coolant chemistry that doesn't break down into acids, so they are
probably not really necessary.

Stuart

-----Original Message-----

To be honest, I really don't see how the little o-ring at top of cylinders
is expected to do much. The combustion sealing ring (in head) should keep
the combustion from the coolant right? If that seal is not good then the
thin o-ring ain't going to hold things back, right? And conversely, if the
thin o-ring fails, the coolant isn't going to get past a good sealing
combustion sealing ring.

I suppose the o-ring keeps coolant from a portion of the head area, but to
what end?

cheers

alistair