A hierarchy (J D g ) D Dilator of ordinal functionsJ D g : On→On is introduced and studied. It is a hierarchy of iterations relative to some giveng:OnarOn, defined by primitive recursion on dilators. This hierarchy is related to a Bachmann hierarchy $\left( {\phi _\alpha ^g } \right)_{\alpha< \varepsilon _{\Omega {\mathbf{ }} + {\mathbf{ }}1} }$ , which is built on an iteration ofg ↑ Ω as initial function.This Bachmann hierarchy $\left( {\phi _\alpha ^g } \right)_{\alpha< \varepsilon _{\Omega {\mathbf{ (...) }} + {\mathbf{ }}1} }$ itself is shown to be explicitly definable by $$\phi _\alpha ^g (\eta ) = g^{\Omega ^\alpha \cdot (1 + \eta )} (0)$$ from a hierarchy of iterationsg α :Ω→Ω for weakly monotonicg: Ω→Ω withg(0)>0. Forg=λη. (1+η) ·ω org=λη. 1, in particular, one obtains as $\left( {\phi _\alpha ^g } \right)_{\alpha< \varepsilon _{\Omega {\mathbf{ }} + {\mathbf{ }}1} }$ Bachmann hierarchy.With every ordinalα≦ε Ω+1 a dilatorD α is associated.D α is “below” the dilator $$\left( {\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle-}$}}{2} + Id} \right)_{(\omega )}$$ , which is defined as $$\mathop {\sup }\limits_{n< \omega } (\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle-}$}}{2} + Id)_{(n)}$$ with (¯2+Id)(0):=1 and $$(\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle-}$}}{2} + Id)_{(n + 1)} : = (\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle-}$}}{2} + Id)^{(2 + Id)_{(n)} }$$ . For weakly monotonicg:OnarOn satisfyingg(0)>0 and $g(\Omega ) \subseteqq \Omega$ , and forη<Ω andα<ε Ω+1 it is proved that. (shrink)