1981 yz125 Update

That is what you call rusted and seized tight, you may have to bang it apart after letting it sit with penetrating oil for a few days but the triple tree and stem may be a lost cause and not savable.

If you get it apart, bypass that whole mess with external hoses. That is, if the frame is not corroded and rotten from years of leakage. After a few hard rides, it will be spraying coolant from them stem area, anyway, as they did when new.

You may end up having to chuck the frame in a press to try and salvage things. Beating on it will guarantee damage to the stem.

Get a torch with a small welding/brazing tip and heat the bearing race. Then knock it out.

Newmann is right-needs a little heat.

I don't know how that stem is made, but if possible you should maybe use a drift that can go through the stem and then you can tap on that instead of the stem threads. I bet if you use a combination of penetrating oil followed by heat it will come apart, that seems the only way to separate dissimilar metals that have corroded.

Screw the nut flush onto the threads to protect them, and use a soft-faced mallet.

And use heat as mentioned before.

Soaking it with penetrating oil for a while beforehand will also help the situation.

Use a Button( piece of round steel) shoulder on it
and a center hole to use a pointed air hammer bit
to drive it out
or a black pipe cap over the threads
But as stated your stem is shot, and will need to be remanufactured,
replaced or both,
your lower frame bore might also need a sleeve pressed in

pits on your stem would cause a leak