  The popular outer gap model of magnetospheric emission from pulsars has been
widely applied to explain the properties observed in $\gamma$-rays. However,
its quantitative predictions rely on a number of approximations and assumptions
that are usually overlooked. Here we examine them, reviewing the main
ingredients entering in the model, evaluating their range of uncertainties.
Usually, in the quantitative applications of the model, key parameters like the
radius of curvature and the energies of the interacting photons are taken to be
a fixed, single value. Instead, here we explore their realistic ranges, and the
impact of these on the consistency of the model itself. We conclude that the
popular evaluation of the trans-field size of the gap as a function of period
and period derivative, is unreliable and affected by a huge dispersion. Last,
the exploration of the possible values for the radius of curvature, the local
magnetic field and other quantities deserve more attention for quantitative
applications of the outer gap model, like the calculation of $\gamma$-ray
spectra, which is the subject of an accompanying paper.
