snipped from digest/ErikO post

>From the corrosion I've seen personally, I see no indication that the
>coolant/water combination or grounding issues are the cause of this
>problem, rather just signs of it. It is only under the headgasket
>sealing surface where this problem is isolated. There are other places

Bingo, it is a simple liquid-solid/galvanic corrosion problem, specifically
called 'crevice corrosion' due to coolant squeezing under the gasket and
sitting against the head. That's why you see that 1/2" wide track of pits
around the entire sealing area. Stats may be interesting, lot of variables
at play here, including what level the heads were torqued down to, what
sort/number of daily temperature cycles is the whole engine seeing, ambient
temps., what age of gaskets, coolant mixture ratio and 'freshness'. I think
the scatter in the survey will be huge, reliability low.

I'd be willing to bet that most leaks occur during late winter/spring, since
very cold/cooled-off engines reduce sealing pressure and allow seepage.
This happens more in winter (plus stiffer rubber), and a flow path ('join
the pits') is more likely to get established due to the large
expansion/contraction motion.
Once a few drips appear, BarsLeak can still seal them, I think this has been
established, but allowing them to keep dripping means the coolant starts to
clear out the flow path and accelerate leak rate.

>you should find corrosion if the problem was one of the two above
>causes. The whole head would corrode for that matter. That fact that
>it's isolated under a surface that is supposed to be sealed makes me
>think of other options.

ditto, I'm not a holistic mechanic, can only comprehend small things
completely, so keep it simple pls. bye, Tim