How much can a 1.8L Miata head be milled?

I don't undertand what milling the head will accomplish.
The mating surface of the head, bolted solidly to the block, would never have contacted any debris or grit, and so there should be no issue with head gasket sealing.
If the flat squish combustion chamber areas on the head are torn up a bit, then use sandpaper or a dremel stone to carefully smooth the rough edges. The damage can't be too bad, if milling .010 - .020 will clean it up.
Milling the head will also increase the Compression Ratio a bit, probably the opposite of what you want to do when adding a turbo.

TBH with decent engine management I don't really see a problem with raising the compression ratio a little, given how little boost the OP is looking for.

The 1.8l NAs aren't exactly high compression engines as they are.

i, also, don't understand why you need to mill the head.

i am still interested if anyone knows the answer, though.

Wonkothesane wrote: The main reason is to clean up the pitted areas in the quench, since there's only two that have decent grooves, and I don't want to hand port two smooth, and leave two the original size. And I'm lazy, so why grind the problem areas when I can just deck the head and be done with it? I know it will raise compression some, but as Boxhead Tim says, Miatas aren't exactly high compression to start with. I was just wondering how high people have went with compression on their NA or lowpressure turbo setups.

Decking/milling won't address those issues, which is probably why we're confused.

As for compression, people boost NBs all the time, and they were had with up to 10:1.

I might be thinking of something else, then... do you have pics of the head?

Wonko - does this look like what you're seeing?

Old thread

The overbore will raise the compression slightly as well. Most people mill the head for two reasons, warpage and to increase compression .

You have to mill a head a LOT to increase compression to any sigificant degree.

You can look at it this way, though - mill the head to clear the pits in the quench pads since that will make you feel better, and then dig out the die grinder and, using the old headgasket as a bore template (make sure it's not massively larger than the bore!) unshroud the valves out to the bore diameter. Will help airflow and bring the chamber volume back up so compression will remain unchanged.

If you wanted to get really anal-retentive (and it sounds like you probably do), get or make a head CC setup and make all of the chambers the same size. And if you REALLY want to get anal-retentive, find out which cylinders tend towards detonation first, and make sure those are a tad larger than the others. You probably won't be able to do cylinder-specific spark and fuel, but cylinder-specific CR is something anyone can do with the engine apart.

I plan on doing this on my Audi mill. Cylinders 4 and 5 will be a little lower compression than 2 and 3, and 1 will be somewhat in the middle. Still debating on if I want to spring for the DIY coatings so I can ceramic-coat the chambers and exhaust ports. And the exhaust manifolds. And the turbine housing, maybe the turbine wheel if Tech Line says that's not an issue. (Yeah, I know it'd have to be rebalanced. Turd-polishing knows no boundaries)