I believe the head on the 617 is cast iron, so I don't believe heating will do any damage.

I would heat it again and try to grab in with a pipe wrench or the equivalent. Hopefully, you still have something to grab.

You may have to remove the oil filter housing to get some more room to work.

As a last resort...carefully drilling out the old plug and using an easy-out tool.

My opinion is that more heat is the answer...It went in...it'll come out!

Good Luck!

The Tenor Man

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Currently Driving.....
1987 Mercedes Benz 300D Turbo - Daily Driver.
2004 Buick Rendezvous...Smaller than it looks!
2005 Mitsubishi Galant - Not bad for a tin box...Lousey Seats
Past Vehicles.....
1994 Dodge B-250 Van- GAS HOG..Succumbed to rust!
1989 Mercedes Benz 260E - Totaled but no one injured.
1977 Mercedes Benz 240D - Loved it, but SLOWWW! - Succumbed to RUST!
1976 Mercedes Benz 240D - Same as above! -
1975 Mercedes Benz 240D - Same as above!