Can't get bolt loose in harmonic balancer! on 2001 Isuzu Rodeo

Tearing this down to replace head gaskets and maybe heads to. Customer got it real hot. Made me a tool to lock the dohc together to lock engine. And cannot get balancer bolt broke loose. Even tried turning wrong way. Just incase it was backward tread. Anybody had this problem? Plus how easy do them heads crack when got real hot.And what size of wrench is head bolts. Looks allen head to me.

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2 answers
GOOD impact wrench!! There is another way but try the impact first. Go ahead and remove the radiator so you have room to work.
I had to set the socket/ breakerbar set with extension pipe and braced it, then cranked the engine over ala key start, bingo loosened right away:)