Lorn, as I'm sure you're aware, most if not all the laws of Physics (as we
understand them) are immutable.

Case in point: electric current through an electromagnet coil (in our case,
a magneto coil) induces a magnetic field in the magnetic structure
(armature) of the magneto. If the current is interrupted (like when the
points open) the field must collapse. It can only do this by forcing the
current to continue to flow until the energy in the field is totally
dissipated. To do this, the coil output voltage must/will increase without
limit, or until it reaches a high enough voltage to arc over somewhere,
allowing the energy to dissipate via the current in the arc. In a perfect
world for airplane engines, that should be the plug electrode.

Increasing the plug gaps too much can force the magneto output voltage (the
so called "spark voltage") to find another path to ground. The insulating
properties of air is pretty good and normally forces the spark to occur at
the plugs - however, the gasses in the plug gap are highly compressed just
about when the plug needs to fire. Get the gap too large and the insulating
properties of the compressed gas in the larger gap may exceed the ability of
the internals of the distributor or the plug wires to hold back a wayward
arc to ground. Fly at the higher altitudes and the insulating qualitiy of
the ambient air in the magneto can decrease enough, relative to even normal
plug gaps, to require pressurized mags to keep the spark confined to the
plug electrodes.

One might be able to use a larger gap than specified when the ignition
system is new but unless you know what your margins really are, you're not
doing yourself much of a favor by pushing this particular envelope.

While I'm on the subject, there are other ways to divert the spark away from
the plug electrodes and the most common is to grab the "cigarette" with
greasy and/or sweaty fingers while R&R'ing the plugs. Grease usually
contains some carbon which provides an easy path for the spark to go awry -
as does the salt from a sweaty finger. These things should never be touched
unless using something like a Kleenex or a really clean cloth and, if I
remember, if necessary cleaned with a swab dipped in alcohol (there may be a
better method but that's what my old A&P told me). One final point, the
cigarettes should be inspected, whenever you're doing such maintenance, for
obvious arc tracking - evidence that they were reinstalled when dirty or
maybe your plug gaps have eroded beyond the maximum and the plug isn't
lighting the fire reliably anymore.

Regards,

Dan Schaefer