This post is prompted by a thread on Woodnet, in which a fellow woodworker asked how wide a mouth on a wooden plane can be and still provide good results. I took this pictures of my user wooden planes (one shop made, two vintage, and a modern premium smoother) to illustrate.
The whole lot of user wooden planes
Mathieson try plane
Mathieson fore plane
Shop made jointer plane
That is a 1/4" wide chisel used as a gauge block. The shopmade jointer (single iron) has an opening of just about 1/16" of an inch. The Mathieson fore plane (double iron but not set up as such) has an opening of slightly more than 1/4". The Mathieson try plane (double iron but not set up as such) has a mouth opening of slightly less than 1/4". I do not have the precision measuring tools (virtually pointless in a woodshop) to measure the mouth on the Old Street Tool smoother, but if I had to guess I'd say its less than 1/4 of the opening on the jointer plane.