Machine shop hot tanked block & head in their block washer contraption. I flushed radiator & heater core both of which had been replaced 25,000 miles ago, new water pump, all new hoses, before re-installation, and refilled with new green coolant at 50-50 mix. Assumed all was clean in water jacket, but not the case.

Less than two years and 2500 miles later I had to pull engine to replace timing cover gasket as I was not able to get it to seal while performing a timing chain advance. Machine shop didn't set right advancement of cam which I didn't discover until cam brake-in. When draining cooling system down, coolant was rather rusty looking. It has never looked that nasty since I got the car in 2008.

Probably I should give the block and head a flush as best I can in the basement first, and than should I add a can of water pump lube & rust inhibitor along with the usual 50-50 mix?

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67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC

Machine shop ‘hot tank’ takes place prior to any machine work. That is good as to get accurate machining the castings need to be clean for accurate locating and clamping.
However the machining process creates chips and shavings that can fall into the water jacket areas.
So after the machining is complete the water jackets and internal oil lines need a robust flushing and scrubbing. With out knowing the exact circumstances it is impossible to know with certainty, but if you did not bottle brush the internal oil lines and re flush and use a strong magnetic probe in the water jacket areas to verify cleanliness, my guess is residual machine chips,,,

I would flush the water jackets, remove the rear block water jacket drain plug.
Put just clear fresh water in the engine for the initial start,
Get it running and run it long enough to get the motor warm and break in the cam.
After it cools, drain the water, by pulling the lower radiator hose and the water jacket drain.
Check to see if any additional debris flowed out. If all is good, refill with mix of anti freeze / water.
I like to use distilled water, who knows what is in tap water these days.

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Doo Ron Ron and the Duke of Earl are friends of mine.....
https://www.youtube.com/watch?v=uX8Nj8ABEI8

Engine has 2500 miles on it since cam brake in. This project was to seal up timing chain cover that I opened to re-time cam machinist set too retarded while it was in car shortly after cam brake in. Fast forward a year and a half. Had to drain her down to pull engine, and discovered it had a lot of rust in coolant. I'm sure he cleaned out the oil ways. This guy builds race engines for the local circle track and drag race guys, always busy with jobs stacked up. By the looks of his shop 90% of his rebuilds are GM.

I did flush block & radiator with water, a lot of rust came out, looking in head behind thermostat it now looks clean so I reinstalling radiator and hoses. Haven't yet refilled radiator. Did not flush heater core as yet, I have to rig up some way to run water through it from a hose without making a mess in the basement.

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67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC