Might have been bronze or tin—I read the book a long time ago! That said, burning gasoline or kerosine can be over 2000°C[0].

[0] https://en.wikipedia.org/wiki/Adiabatic_flame_temperature

steel melts around 1600, the engine would blow up waaay before.

the problem with that story is that if the "seal" is made from a simple material that melts by the engine heat, then it will not seal for long. (or at all.) but likely it's not what Steinbeck wrote.

At what temperature does copper soften enough to squish into a seal shape?

I seem to remember my dad shimming a cylinder that had lost compression with a copper ring; I was a kid so I don't remember any details except it wasn't intended to melt, and whatever it was intended to do worked and we saw the car still being driven around town fifteen or twenty years after he sold it.

it's possible that the car got a replacement part eventually :)

copper becomes ductile between 300-600 C ( https://www.engineeringtoolbox.com/docs/documents/1353/tempe... )

piston ring seals are usually made of cast iron or steel. copper would probably work too for a while, it's good at conducting heat, there's ample cooling in engines, so it wouldn't melt, just wear out very quickly ... and then the engine performance degrades as the sealing gets worse and worse (and it starts to eat oil, soot gets everywhere, exhaust becomes visible), mpg goes down, but ... the car would probably run. (loss of 1 out of 4 pistons is not a catastrophic failure)