Ultimately, this plane doesn't have the stability at the bottom of the frog due to a gap between the frog and casting. The only way it would be stable enough would be to add a thicker iron.
This isn't exclusive to modern planes, but this one has less of a gap than some of the poorly thought out older designs (union planes come to mind, if I recall, and some stanley frog designs have a big gap - I think....I just don't keep the old ones that I don't like).
so, I guess I could cover some of the stability issues by making a custom iron for the plane that's about 1/8th thick.
Casting unbelievably hard, too. I think I may have said that already...if I didn't.
Best fix for the lower part of this plane would actually be to set the frog just behind the casting, carefully file the sloppy bottom of the casting in place and then apply metal-filled epoxy to the small gap between the frog and the casting.
I'll show this in a video. It's deceptive because the gap is small, but it's enough to cause instability with a thin iron and this is what you get:
Notice the edge - that is an iron that planed maybe a couple of hundred feet at most in cherry and maple. I can feel the instability in the plane, but I have a lot of experience in feeling when an iron is vibrating and it's instability vs. just flimsy feeling. In this case, the iron is moving a little bit in the cut, and the result is that even in something innocent that i planed iwth another plane fine - figured hard maple - the reverberation of the iron beats up the edge, dings it and forms a small burr out of the various nicks. This is toxic - an iron that could last much longer transmits the message to the user that it can't last, but without obviously failing immediatley.
When the burr from these little several thousandths defects gets organized (as in, too many of them work in combination along the edge), the plane rather abruptly stops cutting.
Here is a plane iron of my own make used in a type 20 stanley that has support all the way down. This is at twice the magnification so defects would appear twice as large. Notice that they're absent. This plane didn't plane the same wood, but did plane beech and cherry and planed about 800 or 1000 feet to get to this point.
While I think I make the best irons in the world ..
...there's no way that the iron is the differentiator in this case, and the way I'll have to prove it I guess is to throw the new iron in a T20 plane, plane the same things and see what happens.
So, it's a swing and an "almost". It would be an OK plane if stanley could've managed not to make the "Frog better with better machining on the feet" and face, but rather the critical last 1/4th inch at the bottom of the plane, the cost cutting is terminal.
I will probably make a 1/8th 80CRV2 iron for this plane to make up for the poor support at the bottom of the frog and confirm that it holds up well before dumping it sometime next year (also after confirming that the flatness that I filed into the casting remains there).