A Piston is the moving component /part in an engine that is enclosed by a cylinder and is made air-tight by piston rings. In an engine, its purpose is to transfer force from exploding gas in the cylinder to the crankshaft via a piston rod and/or connecting rod.
An extractor collects the exhaust gases from multiple cylinders into one pipe which is then directed to your muffler to be silenced. After the muffler the excess gas is then released out of the exhasut pipe into the air.
A gasket is a mechanical seal (usualy paper or cork) which fills the space between two or more surfaces, generally to prevent leakage from or into the joined objects while under compression.
For example a gasket would be used between your engine heads and your rocker covers to make sure no oil or compression is leaked out.
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