Followup . . . It started with a BANG

[av8]

At Tardel's suggestion, I had the cylinder head surfaced. (There has to be a reason why things happen, such as a head gasket developing a leak.) Sure enough, the machine shop had to remove 0.006 to clean it up -- not a whole bunch, but enough to permit the leak to develop.

In hindsight, I think the leak has been developing slowly for several hundred miles, maybe longer.  I had been having to frequently top up the cooling system. There was no external leakage anywhere, and I attributed the loss of water to a dead old radiator cap; flatheads like lots of ullage -- air space in the top radiator tank -- and get things their way with very active coolant flow from the two pumps pushing coolant out the overflow bib on the radiator filler neck. Anyway, I finally got around to replacing the radiator cap (special order even through NAPA and about $12-$15, so bought one from Sacto Vintage Ford for six bux) and had occassion to check only once before THE EVENT, and it looked as though it had lost some coolant (glycol in the mix at this point).

The combustion chambers in the head indicate some erosion in the affected cylinder (No. 1). Cylinders 2, 3, and 4 still have the tool marks from when the head was domed when new. The erosion in No. 1 isn't as extensive at it appears to be in the photo, and the marks can be seen here as well, indicating that it is simply not in an as-cast condition.

Just guessing at the origin of the problem, but I think the head was not quite true when it was installed during the upgrade to the 4-inch stroke (either a Mercury shortblock or a Mercury crank stabbed into the Ford 8BA).

Any other theories?

[unklian]

Depending on what you are doing, .006" sounds like alot.
But the head will flex some when torqued,
and the gaskets will take up the rest.

Sometimes stuff just happens.

Glad it wasn't real serious.  $$$