Here's a question I havem't had any mechanic explain to me straight.

The engine block is cooled by the water jacket, as is the heads. The exhaust poppet valve is cooled by contact with the head.

Can the same be said of the piston? That's a pretty sizable chunck of metal exposed to the heat of combustion, and only the perhiphery of the piston comes into contact with the water-jacketed cylinder walls.

Don't use dead dinos for fuel:
let 'em rest in peace!

Many diesel pistons are actively cooled with a jet of oil aimed at the underside of the piston. If you have a shop manual for your engine, study the pictures in the rebuild section looking for the little squirter nozzles.
Heat is also conducted through the piston skirts to the cylinder walls. Although aluminum expands faster than a steel cylinder block, the piston doesn't seize when it gets hot. As it expands the conduction of heat across the little gap between skirt and wall improves, and self-regulates to a great extent. The oil film there is very thin, offering little conduction resistance. Keep in mind that the lower parts of the cylinder are never exposed directly to the heat of combustion, so they stay relatively cool, and the skirts are mostly in contact with those cool parts. Aluminum conducts heat well, but looses strength if too hot, so there's an obvious temerature limit. Indirect injection also helps isolate the really hot part of the combustion from direct contact with the piston.
Cheers,
JohnO

Their diesel engine had a two part piston. Both parts were held in place by the gudgeon (wrist) pin. It had an aluminium skirt supporting an iron top.

The design allowed the iron part to be a closer fit into the cylinder. Being the same material as the engine block it expands at the same rate so tolerances can be closer. The ally skirt stopped the shallow iron part from tipping over and saved reciprocating weight.

A clever cyl head, piston top and injector design caused the compressed air to vortex. This spinning air held the fuel droplets and flame away from the metal (just like sugar granules in a stirred cup water). The engine lost so little heat that it relied on oil cooling alone. It did not have air cooling fins or a water jacket system. The design also allowed slow burning fuels (especially vege oils) to be used with little risk of varnishes etc building up on the pistons and rings.

Source: Elsbett web site which no longer contains anything this interesting.