Pistons:
Aluminium Brushed 
Cast aluminium

Piston arms:
Engine block:
Cast Iron

Piston Rings:
Brass
Aluminium Brushed 

Camshaft:
Aluminium Polished
Black Powder

Spark plugs:
Ceramic
Gen Metal Dull
Gen Metal Polished

Others:
Generic Metal Black
Generic Metal Dark Gray
Generic Metal
Chrome
Machined Metal (concentric anisotropic)
Painted Black Metal
Black Plastic
White Plastic
Rubber