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It's part of the gasket. Perhaps it stuck on the head (especially if it was used with a sealant).
Remove it, and you probably should bring the head to some machinist to straighten in on a milling machine, so you again have a smooth gasket surface.
Head gaskets sometimes consist of multiple layers. Such aluminium mesh between the paper gasket layers makes it stronger to resist compression better. It is perforated so the paper gaskets stick to it well. Usually, it is sandwiched between two paper gaskets (and you only got the top off).

I doubt there was any kind of sealant on the head gasket originally. If you have the head straightened (and perhaps the cylinders too, but they do not warp as much), I'd perhaps apply a bit of hylomar onto it (or loctite 518 anaerobic is very good for most things, including where you do not even need a gasket). Hylomar should prevent it from sticking onto the head the next time someone opens it up. But watch out around any oil pathways - better to not use any sealant at all, than to block those (that's why I like loctite 518, it gets flushed away by oil, since it does not harden in air)...
 

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It’s a multilayered gasket and it’s the mid layer to give the gasket it’s resistance to splaying out. The rings next to the cylinder are the sealing bits; as you tighten the head nuts it squashes to provide the seal. When you peel it off you should see a graphite layer which is the other side of this gasket. It’s clever stuff.
 

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Discussion Starter · #6 ·
Oh, wow. I'm new to all this and haven't bought a new head gasket yet, so I had no idea there was an aluminum mesh in between. What's the best way to get the aluminum piece unstuck from the head?

Pikl, is milling it the only option? Could I stone it flat if it isn't too warped? I'm a machinist so I could just do it at work next week, but the quicker option is always preferable.
 

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Oh, wow. I'm new to all this and haven't bought a new head gasket yet, so I had no idea there was an aluminum mesh in between. What's the best way to get the aluminum piece unstuck from the head?

Pikl, is milling it the only option? Could I stone it flat if it isn't too warped? I'm a machinist so I could just do it at work next week, but the quicker option is always preferable.
Well, any way to make it flat is good. I sometimes used some sandpaper on a piece of glass for some of my bikes too (although with separate heads for each cylinder, which is a bit easier to do this way). But a milling machine will make it nice and parallel to the valve cover surface (while sanding it may not be too precise in this way). Never did it this way for an inline engine with a merged head, but it would probably be alright too.

Edit: Actually, I am not certain what stoning means (English is not my main language), but if you mean some form of surface grinding, that's normally an even finer surface finish than milling either way...
 

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Discussion Starter · #10 ·
I mean using a fine oil stone thats larger than the work surface to level it. Luckily, I just checked the head and it seems level- will probably check later with a PG steel block instead of the machinist sq I used, just to be sure.
 

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Don’t worry too much about getting a totally flat surface! The gasket is designed to take up those differences.
 

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Head gaskets don't normally use any kind of sealer, adding gasket goop is asking for trouble later. If you feel you must, because of some scratches in the surface or whatever, spray a thin coat of aluminum paint on both sides and let it dry before assembly.
 
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