they dont have to be. i think it would be more difficult to make somthing with a large gap that can hold a lot of pressure but possible. personally i use a spark that i widened the gap on and it works just fine.
Yes, in fact they do. If you are working on one use a extremely small gap. The comperssed atmosphere makes the spark weaker. You probably didn't notice this reaper because the max you said you used was 20 PSI.
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