You control a nontoken artifact with mana value one or less that can be cast using only colorless mana.
Steps
Activate Krark-Clan Ironworks twice by sacrificing the nontoken artifact with mana value one or less and Myr Retriever, adding ({C} magic symbol) ({C} magic symbol) ({C} magic symbol) ({C} magic symbol) .
When Myr Retriever dies, its ability triggers twice, returning Scrap Trawler and the nontoken artifact with mana one or less to your hand from your graveyard.
Cast Scrap Trawler and the nontoken artifact by paying at most ({4} magic symbol) .
Activate Krark-Clan Ironworks twice by sacrificing the nontoken artifact and Scrap Trawler, adding ({C} magic symbol) ({C} magic symbol) ({C} magic symbol) ({C} magic symbol) .
Scrap Trawler triggers twice for itself dying, and twice for the nontoken artifact.
Resolve the Scrap Trawler triggers for the nontoken artifact, returning any applicable artifact to your hand from your graveyard.
Resolve the remaining Scrap Trawler triggers, returning the nontoken artifact and Myr Retriever to your hand from your graveyard.
Cast the nontoken artifact and Myr Retriever by paying at most ({3} magic symbol) .