Speedmaster Small block porting tips and results Part 1

rednesss said

But even if it is bigger than the bore, wouldn't the benefits of the extra CFM still warrant unshrounding it? The stock bore is 4.04, mine is +.030, so I've got 4.07, meaning .1 bigger, .05 on either side of the circle. .05 isn't that much of a lip. I guess it might create a little bit of turbulence. I don't know, just spitballing here.

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OU812 basically answered this.

ou812 said

I use a Victor head gasket that has a bore size of 4.080 to scribe around the chamber...

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What I’d like to add is if you use a head gasket with a bore bigger than the cylinder and then use that to make the markings on the head to unshroud the valve, there is now a portion of the head chamber larger than the bore leaving a shelf over the deck. This creates a spot for unburnt fuel and/or late burning fuel.

Un shroud the valve as much as you want to within the limits of the cylinder bore. And honestly, that’s way more than enough.