Frank 54 is right, 1/8" is large enough.
This is an old hot rodder trick and works well on a fresh engine that is being filled up with coolant for the first time. Sure wouldn't want a steam pocket to form when you're trying to break in the cam
. Small Block Chevy's are famous for this problem, as decribed above "while driving from a cold start, the temp gauge would go way over to the hot mark (within one mile of driving), then suddenly the 'stat would open and the gauge would drop back to normal."
Frank, I know you were talking about a six, but I have seen this too.
Drilled a hole (1/8") in the 'stat on my '51 GMC. It does slow down the warm up, but it still comes up to 170-180 degrees and stays there.
Overall, a worthwhile trade-off
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