  We consider the influence of extra dimensions on the force in Casimir
pistons. Suitable analytical expressions are provided for the Casimir force in
the range where the plate distance is small, and that where it is large,
compared to the size of the extra dimensions. We show that the Casimir force
tends to move the center plate toward the closer wall; this result is true
independently of the cross-section of the piston and the geometry or topology
of the additional Kaluza--Klein dimensions. The statement also remains true at
finite temperature. In the limit where one wall of the piston is moved to
infinity, the result for parallel plates is recovered. If only one chamber is
considered, a criterion for the occurrence of Lukosz-type repulsion, as opposed
to the occurrence of renormalization ambiguities, is given; we comment on why
no repulsion has been noted in some previous cosmological calculations that
consider only two plates.
