From my 1971 FB.
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There was nothing wrong with the bearing out of my 850 (both replaced with roller bearings)
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I have quick machined bearing guide rings on occasion, the ring sits against the surface, the bearing in the ring then has little choice in starting plumb come time to press it home.
You could use wicking Loctite 290 after the bearings are installed, I would second Bill on the inner seal removed (at a minimum)
You can put the short end of a hex key in that cam plate hole to lock it for bolt tightening.
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Maybe those steering head bearings had some form of Loctite but should have come out fairly easily if driven squarely. Replacements can be drawn in with two OEM Isolastic discs and threaded rod, bottom bearing seated first, then the top bearing drawn in until it seats on the inner spacer tube.
I have to say I can't remember the last PITA moment with bikes in general. If something is not going smoothly, stop and regroup.
Edit.
The large gearbox bearing seems to have a high interference fit even with the case heated so so wonder the case can crack between the two bores. I do not use cooling as it can also produce condensation the moment it gets near something hot, anything cooled would need wiping down before installation to at least minimise that.
I have seen Norton crankshaft main bearing pictures on FaceBook where it looked like they had been left outside the igloo for a month, they were grey !