Originally posted by Aptyp
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the reason is because the head is really thin right in the center of those openings, the head flexes and pushes the gasket out. The gasket then blows and you either have oil/coolant mix or you steam clean the combustion chambers clean/pressurize your cooling system.
BMW actually fixed this on the late M30s. Originally they came with the same coolant passage design as the M20, which had severe issues with cracking and blowing gaskets. Later they were cast solid (like if you weld an M20 head), but they never fixed this issue on M20. Maybe because it's only a big issue when you add boost to the equation. Somehow I doubt the kraut engineers of 25 years ago thought people were going to boost these old cars some day. ;)
Filling in those channels doesn't hurt the cooling capacity at all either. Just looking at how the headgasket is designed makes it pretty clear.
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