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That info you referenced of mine was a copy&paste quote from Ron Black's older posts on this forum.
One could adjust the radius cutter with the head on a lathe so that a tighter radius was cut between the inner edge of the squish band and the top of the chamber nearing the spark plug hole, to reduce compression of a milled head to increase volume, without taking as much from the spark plug thread area top of chamber... But I'd be afraid that'd make for less efficient combustion.

What octane requirements does Millennium give with their kits? That could explain it if they recommend 93 octane etc. I get away with 90 octane non-ethanol (I'm loving Pennsylvania's Sheetz gas stations moving into Ohio with cheap diesel and affordable non-ethanol 90!) just fine on my Ron Black head.

Ron Black states he keeps the head volume *almost* stock in order to avoid the need for race fuel or octane booster (I believe he recommended 91 octane still? Check that though), and to avoid shortening the service life of the engine.

Higher compression will give more torque and low end throttle response, but also higher cylinder pressures and more loading on the big end rod bearings, so the bottom end rebuild frequency may be slightly increased (a lot of our bikes are out there running on 32:1 on stock bottom ends still 20-25+ years later still though, fwiw).

They said '91'..... I told them I have 90 non-ethanol available up here and they said that would work. Alaska doesn't have any ethanol in the gas at all, which is nice (freezes in the winter from all the water in it).
I installed the spark plug into the head last night and it essentially bottoms out with the spark plug threads and the hole threads flush with each other and only the electrode protruding into the chamber.

Chopperpilot, thank you for your private message indicating you'll disassemble your recently Millennium-serviced cylinder.

Ok, got tired of waiting for a 'thermostat' gasket to show up so I made one out of fel-pro material and fired her up! Went through a couple of heat cycles and took a short ride yesterday. Power seems to be way up (shocker!). Of course I haven't done a full throttle anything yet, but 2nd gear throttle will lift the front tire. Carb tune seems like it's pretty close, I kinda copied a TT jetting based on a RB'd 220. I should probably do a re-introduction thread that covers the 2025 version of my bike with the 225 BBK, YZ250 forks, and Gold Valved shock.

Great to hear, chopper!

I started another thread in this section going over the mods on the bike in general. Went for a full ride on Sunday and man, it was amazing. Fuel injection like throttle response.

Done a few rides now. The difference is truly amazing. I know the saying about it's not the arrow and all, but this will definitely increase your riding ability. I went to my practice hillclimb (when I go check my mail, convenient!) and here is where the engine mods/BBK really shine. At the end of last summer I did a ride with a tricky climb to get back. I had a new gummy tire on so traction wasn't a problem, but the issue was the bike would either stall, or wheelie. Well, no more! The bike will climb with very little to no clutch and I can stop on the hill and get started again with the kind of throttle and power that makes it easy to start moving again without stalling or lifting the front wheel.

Chopper, this is a very cool report. Thanks!