Palmgren 84929/Craftsman 6" jointer knive removal

I have a Palmgren 84261 jointer, same as Craftsman 6 1/8" benchtop jointer, its about 4 months old.
Am trying to remove the knives but cannot get the holding screws out There are 4 holding screws per knife and they are listed as 1/4-20 X 5/8" pan head. Think the size listing is incorrect as a 4mm hex driver fits them.
Anyway these things are torqued or frozen big time and will not loosen. If I use much more force I'm either gonna strip the heads or break the hex key. I don't want to bang on them with a screwdriver type impact driver because of the stress that will put on the cutterhead bearings.
Any ideas?
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