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My stove is made with an 8" pipe. It has an 8" chimney as well. I figured the square inches of the 8" stove pipe and sort of backed into the size of the baffles. I took the 2" space between the barrels times to length of the opening with out the baffles times 2, because you have openings on both sides of the barrels. The size of the 8" stove pipe is Pie X R squared. In my case its about 50 square inches, so the opening with out baffles is about 10 inches by 2" so that is about 20 square inches times 2 [ Both sides ] or about 40 square inches total. I decided to try that size because it was close and see how it worked. I had to cut about 6" off each baffle to make it that size, I figured I could always take off more later, it seemed harder to add more back on. The stove seemed to work pretty well, the baffles seem to keep the heat in the oven and the draw seems acceptable to me. Could it be more "Rockety" , well yes it could be, but I am satisfied at this point. It doesn't seem to smoke, it draws pretty well, and I can keep it at 400 degrees to bake my bread. So with your 4" pipe you can have much smaller openings on the baffle runs and still make it draw. Do the math and then see what works for you. Its easy to make changes with this design.

Thanks Guy. I hope to finish this weekend and see how it works.

Good Luck. My 4" rocket stove works well for cooking, I don't know how that much fire will work for Baking. I would guess that you will need to keep fire in it to make the temps high enough for baking. With my 8" Rocket there are many times went just the hot coals are enough to keep it hot enough to Bake. I would fire it with small boards and see how the fire goes. I tried mine with sticks that I picked up out of the yard and found then to be wet, 1/2 rotted and not nearly as good a fuel as split dry boards. The boards made a much better fuel.

I was thinking the same thing, (about the 4 inch rocket stove being too small) but remember my barrels are only 19 inches tall and the inside barrel is only 15 inches in diameter. This has been a great learning experience and I don't have much money in this project. I am teaching myself to weld in the process and that has been fun too. My wife is planning an outdoor kitchen for the near future and I am trying to convince her to go with a rocket oven versus traditional pizza oven. If I can get the small oven to work then I will build another one out of larger barrels.

I would love to see some pictures when you get it finished. I think it should work ok with the 4" pipe. Just try it and see how it will work.

I didn't get as much done as I thought I would this weekend. We have a death in the family - something was able to get into our chicken pen and eat one of our chickens!!!, so I spent most of the weekend building a "Doomsday Castle Chicken Pen." I will take some pictures an attempt to post them. It's been a fun project and I think it's going to work.

So sorry to hear about the Death in your family. Our thoughts and Prays are with you. Hope you had the time to fix the chicken coop so it keeps out the predators. That sucks when you loose a chicken that way.

Sorry to hear that Jim, I went through quite a few coop iterations before I settled on the current one with coils of razor wire and guard towers at the corners. It's like chicken Alcatraz. I kid, but only kinda. Good luck keeping 'em safe man.