azrael

The head and block surfaces from a production block are not usually smooth enough for a solid metal head gasket. Solid metal gaskets have difficulty creating a proper seal between the head and block, and commonly suffer from leaks, unless the mating surfaces are very carefully prepared.

for some more info, you can check out the page linked below, but this is a common issue, and you'll see people talking about it all over the place.

http://www.flatoutgroup.com/gaskets/techtips.asp