I have replaced the cutting iron on a Stanley #4 hand plane (edit: because the old one was kaputt, not by vanity). The replacement blade I got from LV is thicker, at 3.8mm vs the 2.0mm of the old one. The mouth is now too small to allow the new blade to protrude. I measured the current mouth opening in the sole to be 5mm. I am reusing the original chipbreaker, screw, and lever cap.
To enlarge the mouth, it looks like I can file either the front edge, or the rear edge. Is there a right or wrong edge to file?
My plane has a moveable frog (only via the two top screws in the frog, this model has no rear adjuster screw), but it offers only a very narrow range of useful positions. When it's backed out by any more than 1mm from its most forward position, the base casting starts interfering with the mating of the blade against the flat surface of the frog.
That gives this frog a total range of front-back adjustment of about 1mm, if I understand correctly. When I set the frog as far back as allowed (ensuring that the blade mates well with the frog), I can't lower the cutting iron past the mouth -- the cutting edge of the blade bumps against the fore edge of the mouth.
Frog backed up completely. I don't understand how this position would be useful in any way and I question the design. There is about 2.5mm of material I could file before the black rib between the two legs of the frog would start interfering with the blade.
The blade hits the sole before clearing the mouth. In this image, the frog is set as far back as possible, without the base casting interfering with the mating of the blade against the frog.
Rob Cosman, in "Filing the Mouth of a Plane" shows how to file the fore edge of the mouth without mentioning the back edge.
Christopher Schwarz, in the book Handplane Essentials (in chapter Metal Bodied Jack Planes), writes that you want to be able to configure a mouth opening to be 1/16" (~1.6mm) for general work, or smaller (for finer work). He describes also that the black rib between the legs of the frog can be filed if necessary, but doesn't describe a strategy for filing the edges of the mouth.