Dangers of subjecting engine to over temp

Question

It was explained to me long ago that if an engine gets too hot, it's at risk of seizing. The reason, as I understood it, was as follows

1) There's a certain gap between the piston and bore. As both increase in temperature, this gap decreases.

2) In the absence of sufficient cooling, this decreased gap will result in more friction, generating more heat, which results in more expansion and friction, etc...

3) Eventually, the gap will be small enough and the temperature high enough that the piston will 'cold weld' to the bore.

I'm revisiting this explanation and starting to question the validity of it. Assuming the piston and bore are made of the same material, won't they grow at the same rate? Why should the gap decrease as temperature increases?

Answer 1

It's usually the rings which seize in the bore first before the piston. The piston will expand some when heated, but the rings will close their gaps. My understanding is, as soon as the gaps touch, they'll stick in the bore and cause the pistons to stop, as well as the rest of the rotating assembly. Once they've cooled off sufficiently for the gaps to clear, most often the engine will unseize and you can rotate/start it again. You can believe there is plenty of wear on both the bore and the ring when this happens. This can happen quite readily if your engine is forced inducted (turbo/supercharger) and the rings haven't been clearanced to handle it.

As far as the piston and bore, they aren't made of the same material (for the most part). Pistons are usually made of aluminum, while the bores are made of cast iron (will most likely be cast iron sleeves if in an aluminum block). There have been some aluminum bore engines before, but they are far and few between. These materials (iron/aluminum) will expand at different rates.