The 7' machine has me slightly puzzled lately and Im not sure quite what to tell you. The first one e made involved N35 grade magnets and the mechanical clearance (what you are calling airgap) between magnets, and stator) was reasonable at about 3/32. With N42 grade magnets I find it can be as much as an 8th or more to keep the cutin reasonable at about 200 rpm. We made one with smaller, rectangular magnets, still a single rotor and it seems to work pretty well, better than I'd expected, the gap is about 1/16th (A bit tight) but the cutin speed is fine. It does run kind of fast thoughl.