GM aluminum 842 heads open chamber?

[Igosplut]

Originally Posted by markjohnson

The problem is that the expansion rate of aluminum is greater than the expansion rate of the steel valve seat insert. The aluminum head expands and the valve seat just falls right out if the machining tolerance isn’t dead on. The high end solution is copper beryllium valve seats that actually expand more than the aluminum so they really lock themselves into place.

Funny you say this, as I had a valve seat fall out of an older Harley a while back,. and the machine shop used a copper/style seat to replace it, stating the very same reason.