Spike,
The gap will be .076" or 1.93 mm. The tube sizes are as follows:
1" o.d. x .049 wall
3/4" o.d. x .049 wall
1/2" o.d. x .035 wall
I figured I would go with a larger gap size due to the neutral tube and the statement that Karebill made about loosing gas production when his pump was not running.
My other cell (standard Stan Meyer set up in 4" o.d. tube) will have a smaller gap of .060" or 1.5 mm
LapperL