Yep, tiny shiny specks of aluminum in the coolant. The pump is a 4 year old electric Proform pump on a mild sbc. The pump had a small leak at the weep hole so I yanked it to be replaced by a higher flow pump. I pulled the pump apart and see nothing obvious that could shed aluminum. The impeller turns freely with no excessive play in the bearing. Any other guesses at the source? Btw, the oil is clean and free of metal or debris.

Was there anything new at the beginning of this build? Radiator, intake, heads, etc... Anything aluminum that could have had machining chips in it when it went together?

Exactly what I was thinking. Someone maybe didn't wash those heads very well......

The Super Victor intake was new, but I suspect the pump may still be the source. I didn’t separate the impeller from the shaft so there could be something there. No biggie. Just puzzled.

Got it. When generously porting the aluminum heads with burr bits and sandpaper rolls, some of the dust and bits wound up in the water jackets. Plausible, eh?

If the impeller blades were rubbing on the inside of the housing it could make some debris but I wouldn’t think flakes. I am not very familiar with that pump but if it’s like most pumps the back side of impeller is flat and that is the side you haven’t inspected then I doubt you will find anything back there. I have known a few people to have their pumps running backwards and impeller backed off the shaft till it started rubbing the housing otherwise the impellers spin in direction that would tighten it not loosen it.

I have a profits pump in my garage I am repairing for somebody and will have a look but if memory serves me correct the impeller is held onto shaft by a set screw.

I also doubt the back of the impellor. If it was rubbing you would hear it if you dry run it for a second.

I think if the impellor was rubbing it would make big enough flakes to see.

Likley some defris left over from the heads getting machined or the radiator being built.