hey guys. For my '72 blazer, I took an engine out of an '80 scottsdale. I had it bored and I have just finished work on the heads. I noticed that this is NOT factory engine for that 80 truck. The stems on the exhaust were larger than the intake stems. I called the local shop and they tell me its because the exhaust stems are filled with liquid sodium. They say that is sloshes around in there and helps keep them cool. Anyone know when sodium valves were introduced by chevy?