Trying to cast engine pistons in a high Silicon alloy.

Got some ingots of A390, which is meant to be the best alloy for that, but all of the pours have black air pockets near the top, that seem to carry quite a way through the casting (i.e. unable to machine away to get clean metal)

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The inside and bottom of the casting are shiny and beautiful.
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They are currently being done in a primitive split die, with a side pouring tube that feeds about a third of the way up:
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All the dies are well pre-heated.




We are at a loss as to why, have tried a few different top plates (this is the second one, with a smaller vent hole, and a taper inside to try and float any impurities up)

Interestingly, the ingots we bought also have the same "feature":
IMG_2707.jpgIMG_2708.jpg
as does a random dribble that was dropped on out base plate, and I machined the top off.





Here is the last pour from yesterday. Got the alloy extra hot - note the ripples on the surface from the oxy torch:
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(haven't dissected the casting yet, but I suspect it will have the same issues)




So, apart from "just sand cast it", does anyone have any suggestions?
Attached Images




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