  We consider minimal 5-dimensional extensions of the Standard Model
compactified on an $S^1/Z_2$ orbifold, in which the SU(2)$_L$ and U(1)$_Y$
gauge fields and Higgs bosons may or may not all propagate in the fifth
dimension while the observable matter is always assumed to be confined to a
4-dimensional subspace. We pay particular attention to consistently quantize
the higher-dimensional models in the generalized $R_\xi$ gauge and derive
analytic expressions for the mass spectrum of the resulting Kaluza-Klein states
and their couplings to matter. Based on recent data from electroweak precision
tests, we improve previous limits obtained in the 5-dimensional Standard Model
with a common compactification radius and extend our analysis to other possible
5-dimensional Standard-Model constructions. We find that the usually derived
lower bound of $\sim 4$ TeV on an universal compactification scale may be
considerably relaxed to $\sim 3$ TeV in a minimal scenario, in which the
SU(2)$_L$ gauge boson is the only field that feels the presence of the fifth
dimension.
