Use a hammer and drift to shock-load the plug. Make a larger tool that fits a snugly & use a hammer to firmly tap while putting a torque on it.
Hearing might work, as it will expand the ally quicker than the steel plug.
Loctite sell a 'freeze-off product that works a treat & will probably shrink the plug. I would be surprised if this doesn't work. Last resort I'd weld a socket or decent sized bolt to it. Keep a extinguisher free.
Once the plug is out, repair or replace it & use loctite 567 thread sealant.
Reply With Quote