  In the framework of a Kaluza-Klein type theory with the Standard Model fields
localized on a 4-dimensional section while gravity propagates in a full
$4+\delta$-dimensional space-time, we examine a mechanism of naturally small
neutrino mass generation through couplings of Standard Model singlet fermion(s)
living also in the full space-time. A numerical studies is carried out on the
charged current universality constraint from the ratio of pion decay partial
widths. The bounds obtained on the fundamental mass scale could be stringent.
