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Can you not make them if you had some copper?
My Chinese 1/4 x 32 spark plugs actually measure 6.1mm OD, a bit short of the theoretical 6.35mm = 1/4". So with a little luck and a following wind, a 6mm copper washer may just fit. I guess a tight washer could be stretched (expanded) with a tapered drift.
Great result that easily can be copied by others Per