Goblin Welder + Corridor Monitor + Dross Scorpion + Liquimetal Torque

Goblin Welder

Corridor Monitor

Dross Scorpion

Liquimetal Torque

How it works

1. Activate Liquimetal Torque's second ability by tapping it, causing Goblin Welder to become an artifact in addition to its other types until end of turn.
2. Activate Goblin Welder by tapping it, sacrificing Dross Scorpion and returning Corridor Monitor from your graveyard to the battlefield.
3. Dross Scorpion and Corridor Monitor trigger.
4. Resolve the Dross Scorpion trigger, untapping Goblin Welder.
5. Holding priority, activate Goblin Welder by tapping it, sacrificing Corridor Monitor and returning Dross Scorpion from your graveyard to the battlefield.
6. Resolve the Corridor Monitor trigger, untapping Goblin Welder.
7. Repeat from step 2.

This clever loop turns your graveyard into a workshop by using Liquimetal Torque to temporarily make Goblin Welder an artifact, allowing the Welder to swap itself back and forth with a sacrificed artifact creature. As Dross Scorpion and Corridor Monitor trade places in and out of the graveyard, the Scorpion's death trigger untaps the Welder while the Monitor's entry trigger untaps it as well, keeping the metallic engine running indefinitely.