on my 6" diamater reactor I did not use any groove for the oring on the flange. I can confidently say that most of the people that have built a reactor on this board and talked about it and are not lurkers :P have not used a groove as well.
I just looked up orings in the yellow pages and mesured the size I needed and went to get it and I got 2 for free. I guess it was too much work to write up the paper work for 2 orings
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No matter what the morrow brings, inventors keep inventing things.
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Jonathan
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