Sacrificial torx key and Wd-40 sound like a good place to start. If needed, my metal shop could help.
Use plus gas or the WD release agent.
Slightly larger tx bit than the Allen key and the turn gently.
Ally stem and steel bolt, I'd pour a couple of kettles of boiling water over the stem and bolt to see if differential expansion breaks any corrosion between the two
WD release agent
V good point. The fast release penetrant is worth every penny.
@lynx started
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Sacrificial torx key and Wd-40 sound like a good place to start. If needed, my metal shop could help.