what does it do to the aluminum?

You have aluminum pistons?

The ill fated Chevrolet Vega had a 4 cyl aluminum block - no sleeves. The rings rode directly on the aluminum. I believe the walls were impregnated with something like silicon which prevented wear - unless the block overheated. Unfortunately GM elected to forego a $5.00 coolant recovery kit, and of course the block overheated and the two middle cylinders would melt.

I thought it was a beautifully designed engine. I could disassemble and remove the entire engine except for the cam in under two hours with hand tools. You didn't need a lift because the aluminum block only weighed a few pounds. And it ran well.

I believe it was an early form of alusil, like in many Porsche engines. These still use a aluminium piston but it has iron impregnated into the surface.

The bore is what is impregnated with silicon, the pistons are bare. You can moly coat or DLC the pistons, but that is not what Porsche has done. They do nikasil on the bores.

Pistons, Rings and Coatings
Years ago, GM used a hypereutectic alloy in their Vega four cylinder engine. That engine had its own problems but rebuilders were left in the dark as to how rebuilding should take place. Given the relatively high tech nature of materials and machining vital to restoring wear surfaces in these cylinders, rebuilders experienced failures when employing tried and true methods used for cast iron blocks. Even when correct surface generation processes were employed, failures continued. The fix then was to sleeve with cast iron liners and install standard aluminum pistons and garden variety cast rings.
So what was missing in this exercise? What eventually became clear was that piston skirts had to be coated and ring faces should be barrel shaped if Alusil cylinder bores are to be retained as a wear surface. When this news finally hit the rebuilding industry, rebuilders already had a workable fix by sleeving with cast iron liners. But that fix is not always possible with modern engines using Alusil as a sliding seal or wear surface.
Early in the process, piston coating for use in aluminum cylinder bores became know as “tining”. As it turns out, “tining” piston skirts was indeed a thin layer of plated tin used as a wear or scuff barrier between aluminum pistons and aluminum cylinders. “Tining” then became a euphemism for several piston coatings that acted as wear barriers. Some manufacturers used a variety of coatings that included nickel–tungsten (Ni–W) plating, electroless Ni plating, Ni–P coatings with ceramic particles such as boron nitride (BN), SiC, or Si3N4, as well as titanium nitride physical vapor deposition (PVD) coating, diamond-like carbon (DLC) coating, spray cast iron and hard anodizing. Not all of these coatings proved reliable against scuffing Alusil cylinder bores and were eventually replaced by more robust piston coatings we see in use today.1
Due to the abrasive nature of Alusil and the affinity aluminum has to itself; this coated barrier greatly reduces seizure of component parts. A stock aluminum piston would simply not survive even in a well prepared Alusil cylinder bore. Further, barrel shaped ring faces were used to glide across correctly honed and etched silicon particles without dislodging them. Beveled ring faces utilize a scraping action which may be detrimental to an Alusil wear surface. Exercise caution when selecting a suitable ring pack for use in Alusil bores.
Further, many different ring materials are used for Nikasil® and Alusil but some of the better ones are Gas Nitrided, Ion Nitrided or titanium coated tool steel. There appears to be some consensus that using a barrel shaped ring for either Nikasil® or Alusil cylinder bores works best. However, always go with the coating or cylinder alloy manufacturer’s recommendation for a suitable ring pack.

You have aluminum pistons?

I have aluminum pistons. Probably in every car I own.

I use a wirebrush to clean the tops and an old ring for the grooves.

and what magic set of pistons do you have?
I believe everything I own, except maybe the old Volvo's, have aluminum pistons.

rich

No....

Nikasil pistons are bare, alusil pistons have a coating, "iron coating" is probably a bad technical term, but is one a layman would understand.

I, quite honestly, had no freaking idea they could even be made of aluminum and withstand the temps inside an engine. Then again I've never really gone that deep into an engine, other than American stuff. I clearly still have a lot to learn from you Rich.

I, quite honestly, had no freaking idea they could even be made of aluminum and withstand the temps inside an engine.

Interesting. I grew up with farm equipment, some of it really old, general mechanic starting in the late sixties rebuilding cars and trucks from the forties and fifties, and in all that I've never seen a non-aluminum piston. This thread got me thinking, I'd just assumed all pistons were aluminum, has anyone actually seen anything else?

Yep. On my old Thunderbird and many old 5.0s the pistons were steel. I could have sworn the screwed up piston that came out of my dad's bus when I was a kid (also doubled as his ashtray) was steel... but I wouldn't have known any better then. The pistons that were in the 96 Chevy Cavalier that I was working on a few months ago sure seemed steel as well (rust on the tops since the car say with a blown head gasket for some time).

Nope, aluminum pistons, steel rings and steel sleeves as cylinders.
Aluminum cylinders do work, porsche uses them with a chrome (steel) coating.

So, the rust you see after a head gasket blowing out is not from the piston, it's from everything else.