Haven't seen reliable info in print on the combustion chamber volume of the 848 head. I cc'd a 848 head yesterday.This head had a .008 clean up mill.Appeared to never have been milled before.Fresh valve job,stock intakes,stock exhaust valves (SBC actually)with new seats,extended core Y type Champion spark plug. The combustion chambers averaged 79 cc. I checked the number one chamber twice to be sure. So if you zero deck a 261 block with flat top pistons,use available head gasket with .050-.055 compressed thickness,the engine will have around 9.1 compression with a .060 over bore. Zero deck pistons might require a slight intake valve clearance pocket that'll reduce compression about a tenth of a point.
An unmilled head will be exactly 4" thick and your numbers appear to confirm this quote from Inliner's: "A few years ago, Tom Langdon, before he retired from GM, was kind enough to provide me some combustion chamber cc's based on casting numbers: 3836848 79.1 cc's 3835913 86.2 cc's 3836850 86.2 cc's 3703570 95.5 cc's"
On the bottom of the heads I've seen, on the end is three small casting numbers.These will be ground down by milling.
That pad is definitely a good visual indicator of head milling and is shown and described in this tech tip: http://www.stovebolt.com/techtips/milledheads.htm However, accurately measuring overall thickness and subtracting it from 4 will tell exactly how much has been removed.
I have those exact same numbers written down and I got them somewhere online, I don't think it was Inliners. From the heads shown in the list the 848 definitely is the high compression head, it would be helpful if folks with other 18-bolt heads would cc them and add them to this list.
I am personally responsible for linking that information a second time at Inliner's as well as spreading it to Stovebolt.com and now here and am sure it exists in other places too. I can add that removing .075" from an 848 head will completly remove the "quench trench" at which point the amount removed relative to volume will change drastically. In addition the intake valve stands proud about 1/16" at that point.
I measured the head as Curt mentioned..... .008 short of 4 inches,exactly what was milled off. The quench trench,I'm no expert tuner but if 75 thousand is milled off the head,the intake valve is generally sunk in to match.The "short side" radius of the intake port bend to the valve seat is already too short for good breathing and sinking the valve makes it worse.If the valve isn't sunk a deep pocket is needed in the piston that may mess up air flow.With very slightly milled head and using a cam with moderate overlap then a valve relief in the piston of about .060 is needed on a zero deck engine for a margin of safety. But it's all just theory without back to back comparison testing.
My experience has been that even after altering the short side turn from 1/8" to 3/4" (which took some creative toolpath and a custom indexable cutter) there is still room to push back the intake valve 1/8" before interfering with the turn if one so desired. When I get to investigating that I intend to move the valve as little as possible as the compression sacrifice seems to negate the whole purpose.
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