.......this one goes to 11.

Why not make 10 the highest?

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If it was easy, everybody would be doing it.

Haha that's definitely an idea. Would I need a new cam if I cut off 0.090?

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1980 Plymouth Volare Duster w/ 225 Slant 6

Step 1: Buy a head measuring kit like this one. You might be able to find one cheaper. Use it to determine the volume of the combustion chambers on the head you plan to cut. To ensure an accurate measurement, clean the business side of the head well, and then use a little bit of grease around the perimeter of each chamber to seal the plate against the head so you don't leak water out. Once you have that number (take the average of all 6, or use the smallest one to be conservative), figure out what the chamber volume would be after milling however much. Like Josh mentioned earlier, you lose 1 cc of combustion chamber volume for each 0.0066" you cut (up to a point). So cutting 0.060" will drop this value by about 9 cc.

Step 2: Plug all the numbers into a static compression ratio calculator like this one. You will need to make some further assumptions, but here's what I'd use if you're sure this engine's never been apart before:
Bore: 3.4"
Stroke: 4.125"
Head Gasket bore: 3.5"
Compressed head gasket thickness: 0.040" (average for Fel-pro)
Combustion chamber volume: [your numbers]
Piston dome volume: 0 (for stock flat top pistons)
Piston deck clearance: Here's where things get fuzzy. For an unmolested 60s-era 225, this is usually around 0.180". For your engine (1980?) I'm not sure what's typical, but it might be a little less. Others on this forum might be able to help here.

Step 3: Once you have a calculated Static Compression Ratio, use a dynamic compression ratio calculator like this one. For this, you'll also need the rod length (6.7" for a 225) and the intake valve closing point in degrees After Bottom Dead Center (ABDC). Others on this forum can tell you what this would be for a stock cam, once you confirm what year your engine is and whether it's mechanical or hydraulic.

Once you have a DCR for your target head chamber volume, bring it back here and we'll talk about cams (and fuel, and ignition timing, etc.). My guess is you won't want to go much more than 0.060" with a stock cam on pump gas, but that isn't a well-informed opinion... yet.

I had about 0.085" cut off my stock head, and even running a more aggressive than stock cam I was burning through head gaskets like they were going out of style. Granted I was running 87 octane, had no idea what my ignition advance looked like, and was using the wrong spark plugs because I had no idea what I was doing... don't be like I was.

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Somehow I ended up owning three 1964 slant six A-bodies. I race one of them.
Escape Velocity Racing

.090" with a stock cam will get you into possibly needing shorter pushrods.

Bottom line is that unless you measure, measure, and measure you will have no clue what you have.

Me personally? Unless I was going to do a whole build I wouldn't bother cutting the head other than making sure it's flat.

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Official Cookie and Mater Tormentor.

Stop by any local Vet and pick up that syringe for about $5 and a piece of plexiglas at the hardware store for about $1. Your prices may vary!

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2 Mopars come with Spark plug tubes. One is a world class, racing machine. The other is a 426 CI. boat anchor!

12.70 @ 104.6

I use a CD cover instead of plexi......$0

I swapped a head that had been cut .080" onto my '67 Valiant and it was okay with pump premium. It was a static 8.5:1 with the stock cam. It might have been okay with mid-grade or regular if the ring seal had been better and passed less oil into the combustion chamber. The head also had larger valves and was accompanied by intake and exhaust upgrades.

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Joshua

I'm with 'zilla, whack it .060" and bolt it on, or just clean it up for a flat deck.
More important is a quality valve job, for performance and efficient operation.
Any more and You had better do as advised above,..measure measure and
measure, and have a comprehensive game plan.