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I was wondering if there is a max/ min on the side gap in a metal drum.....If it is stated somewhere , I have apparently missed it.....

It would vary with system size. You'll want to keep a consistent volume throughout the system, so you'll have to measure the outside dimensions of your riser and inner dimensions of your barrel or outer covering and subtract the area of the riser from the barrel. That remaining area will need to be at least your CSA, and the larger the better. Or more simply, probably the same as the top gap, so minimum 1.5" for a 6" core and 2" for an 8" core. Again, the bigger the better.

Obviously, the opening to the plenum can't be 360* around the core, so the gap will have to accomodate that area, too.

Thanks Manny,. Just wondering about a 10" O.D. in a 22" 55 gal drum....on a 6" system.....3 1/2" side gap....I am considering making a 6" batch box...The 4 incher I made seems to work well with the 8" O.D. in a 14" drum....3" side gap...trying to gather materials.

Your side gap would be more than sufficient, but I would try to insulate the riser more than that. 10" O.D. on a 6" system only gives you 2" of insulation all the way around. I would go for more, not to mention once that inner pipe on the riser burns out you're not going to have much structural integrity left with 2". On my old 6" system I had 14 O.D. stainless pipe for the outer riser form, which gave me 4" of insulation all around the riser, just like the core. This might be more than necessary but it worked great. And I still had oodles of room in that 22" drum.

Manny nailed it TS. No such thing as too big a side gap in my opinion, but depending on what you are casting with, I agree that 2" wall thickness isn't enough if it's a diy soft-ish mix. If it's castable refractory, it's probably enough to be strong, but I'd still want to wrap it with superwool or something to get more R value.

I am also thinking of using some 8lb' cal sil 2" board and making a square riser.....with maybe some ridgidizer as I wonder about direct flame contact after a while....

That's an easy way to build 'em TS. There is a bit of warping and pulling to deal with, but for the most part it works well. I have seen one after a year and it was fine, the inside burns out the binder and creates a fluffy layer that protects the rest of it. Should last indefinitely, in my opinion.