Coeficient of linerar expansion.

[jannes_z-28]

I had that problem on an Opel with the 2.0 Ecotec engine. The explanation was that the manifolds bolt hole were a little to tight and when hot expanded and pinched the bolt and pulled it. Eventually the bolt would snap at the head. As mine did, the mechanic were able to repair it without pulling the engine. Small engine, big car = lots of room to work.

GM was aware of the problem and on later models they made the hole in the manifold slightly larger so that the bolt would not stuck in the hole.

Jan