It's not. The size of the hole is the issue, it's the dimensions. Before plastic grey knights came out, if you sliced the snout on a terminator helm at angles, you could very easily get a helm of the right size for making your own GK helmets for Power armour and terminator armour both. You can use power armour heads inside terminator armour receptacles, but... Not without filing. I see a potential issue with the crown on the head, but, some filing around the back, or resculpting the back with some GS should see it work fine. When in doubt, try GK armour. That'll take more work to deal with to make it generic, but I'm pretty sure the have fewer head issues
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