How critical is the back wall 1/8 gap? What wil it cause if bigger?
The ⅛ gap refers to the mortar joints. You want to keep them as thin as you can to make sure the surface is as much firebrick as possible. The wall is not like the arch; it’s a straight vertical wall. That means it is not self-supporting the same way the arch is.
As for what might be triggered, you might lose a little heat, and larger joints increase the risk of cracks in the mortar. The natural thing would be to make larger horizontal joints because you just came off working on the barrel where the joints are enormous on the outside. But if you look closely at the INSIDE you’ll see they are still quite narrow.
Like everything else with this project, don’t obsess if you got them a little wide. Just make sure you have sealed the spaces thoroughly so that hot gas can’t leak through, and remember that the exterior of the barrel will be covered by the time you finish.
Thank you I will add a extra layer of ceramic blanket. You ar right I thought I was going thin but not thin enough.