SpottedLantern
Newbie
As someone who recently went through this, I am not picking nits here. I want to make sure I'm plugging in the right numbers. I believe where you write "radius", you're referring to Segment Height. Since you mention "20-inch diameter", I used 10" for the radius.
Working your numbers backwards since I think you're asking "Which is correct: 51.75 or 75 sq in?" - and - "How high should the segment I'm removing be?"
Using the formulas in this post :
51.75 would be correct if the inside length of your tank is 41.25"
Segment Height = 4.375" (4-3/8")
Chord = 16.5"
to achieve an area of 50.8 sq in... almost 51" (another 1/8" to 4.5" Height with a slightly longer chord puts you at 52.9 sq in).
75 sq.in. would be correct if the inside length of your tank is closer to 60"
Height = 5.75" (a number you already mentioned so I'm guessing this may be the length of your cylinder.)
Chord = 18" (18.1" if you're better with the cutout than I am.)
If you cut too much all I can say is, "No big deal. You're obviously handy and secure enough in your skillset that you even started this project instead of running out to a BigBox store." So two options immediately come to mind:
1. Weld in a piece of flat along the bell to lower the firebox, or
2. Cut the top of the corresponding smiley-face mouth in the back of the firebox down a little lower to make it smaller.
Which option you choose will be based on your desired outcome:
Do you want the firebox higher off the ground but subsequently closer to the grease plate and cooking grate? - or - (cut the firebox opening to the corrected size)
Do you want the firebox further away from the top? (add the flat to the bell).
If I'm way off-base or coming across as anything other than trying to help a brother out, I apologize in advance.
Regards.
