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I have some concerns about choosing materials for my power piston and the cylinder.

I have thought of using Teflon pistons on cast iron/steel based cylinder. Any other recommendations using other combinations? (metallic-metallic which I have problems thinking of one) Thought of using a brass piston and I don't have much idea what material the cylinder will do well.

Secondly is the clearance which im going to make is about 20 microns (0.008") between the piston and the cylinder wall for a good sealing of the chamber gas. Is it sufficient as a sealing method or any other ways.

A graphite piston with a copper, steel, Pyrex, or brass cylinder. Tighten up your piston to cyl wall clearance and test fit. You can make the piston smaller diameter for a smooth fit but once undersize....its all over for that piston.

blitz000, for a metalic piston cylinder combination, cast iron piston , cast iron cylinder, next is C I piston and a steel cylinder (the combo that I usually use,can be mild,med carbon, or stainless).The graphite, and glass is the best, but the CI on CI is quite good. Teflon on steel (mild/ med carbon or stainless),is OK if you can manage the expansion rate, it is possible, my smallest motor has a teflon sleave on the piston. Bronze is better than brass, and you can buy your cylinder off the shelf in the form of bronze bearing bushes, or you can selvage old bearings from old machinery, but the old ones will require some work to clean up the bore. Ian S C

In my opinion is good, (with this range of dimensions and temperatures (low)) to use Rulon sheets as ring seal on piston or Teflon coated needle, all on stanless steel (very) thin piston, for cylinder is good stainless steel, being the very polished surfaces are good for Teflon product, without the iron-carbon roughness. or you rely on the lubricating effect of the carbon, or on the Teflon composites.

More tradictionally the self lubricating effect is done by bronze, (good), or bronze-graphite loaded (very good) or bronze-graphite sintered (very very good), for piston.
Carbon iron or stainless steel for cylinder.
The bronze is at all the best for anti-friction material, as it is, a lot more than other, ( a lot more than brass or not hardened steel), bronze may be is recovered from dismantled machinery, used for bush, for so may be is yet in good anti-friction alloy.
Is not said that all piston have to be in bronze, may be is only a ring in friction area, the rest of the piston can be in stainless, thin, resilient.

Of course bronze is the best for hard metallic material.

(PTFE + aluminium) is great.

If you want to use aluminium, it should be hard anodised, this system is used on some 2 stroke motors, it was also used by Andy Ross on some of his hot air engines.
Lowest friction, metal to metal is cast iron, on cast iron, next is CI on steel. A bronze clinder witha steel, or CI piston would come next, you can proberbly get away with it a wee bit thighter than the other two options, because the bronze expands a little more (very little). Ian S C