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Pretty common problem... Debris in the cooling system doesn't get properly cleaned out by EPA compliant hot tank.. It just gets loosened so when the engine is assembled & running in the car
after a few heat cycles the debris starts breaking up & plugs the radiator... A good solution is to run a coolant filter for the first couple thousand miles...

http://autocoolantfilter.com/

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In a new motor, I agree.

I spend hours MYSELF just picking the loose scale out of my block and air blowing the living heck out of it! And it was real clean to start with. Then after I got it back from the machine shop I put a pressure washer and hose through everything.

My new motor stays cool and I have a tiny mechanical fan with no shroud yet. I did do a drain and fill soon after I got the motor running. But if it idles or in real slow traffic for quite some time it will get hot (not boil over though). I need to add a shroud AND do a good backflush.

I thought I did a decent job of getting the junk out of there