[tet68]

Tapping on the heads creates vibrations that pull the
soaking fluid into the joints of the threads, don’t be
in a hurry, work, then let sit, spray and repeat, leave
over night if you can. Trying to drill them and use an
ez-out may be the last resort but be sure to use the
correct size of drill and EZ-out. Good luck.
Make sure your allen wrench is the correct one and a
good solid fit. Tap on the wrench as you try turning it
too.

[Windflier]

Tapping on the heads creates vibrations that pull the soaking fluid into the joints of the threads, don’t be in a hurry, work, then let sit, spray and repeat, leave over night if you can.

Thank you, Tet. That's just the sort of mechanic's advice I'm looking for.