Well, replaced the thermostat today, flipped my fan over to the correct side (again, never had an overheating problem since it was off last dec when I replaced head gaskets.

So, the original topic being bad radiators, but bad because of cracks. Should I suspect the radiator, even if there are no cracks? What is the process of deciding wether a radiator is good or bad? Just elimination?

The truck today, on start up after thermostat replacement, I had the heat on high while I filled the raidiator, the gauge read halfway, but I had no heat. So I drove it around the block a couple of times, gauge still pointing down halfway (normal position) then all of the sudden, I had FULL BLASTING HEAT! so drove a bit further, and the gauge started creeping up to the hot mark turned around to drive back to the house, ( this whole time I have heat on high) and the gauge starts to creep back down and staying at the normal position. So I continue driving a bit more, but I turn the heat off, and temp gauge starts to climb again, turn heat on high, gauge goes down to normal.

Does any of this sound like a reason to suspect the radiator?