Went to change the thermostat in my wifes car and broke the head off one of the two bolts that hold the cover on. There's enough left to grab with vise grips and I've been soaking it with liquid wrench and banging on it but it won't turn. Is there anything else I can do besides keep at it and pray?

BTW the liquid wrench is 20 years old, does that matter?

TIA

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5 speed '91 190E 2.6 320,000 mi. (new car, fast, smooth as silk six, couldn't find any more Peugeots)
5 speed '85 Peugeot 505 2.5l Turbo Diesel 266,000 mi. (old car, fast for a diesel, had 2 others)
5 speed '01 Jetta V6 (new wifes car, pretty quick)
5 speed '85 Peugeot 505 2.2l Turbo Gas 197,000 mi. (wifes car, faster, sadly gone just short of 200k )
5 speed '83 Yamaha 750 Maxim 14,000 mi. (fastest)
0 speed 4' x 8' 1800 lb Harbor Freight utility trailer (only as fast as what's pulling it)