  An algebra $\mathbf{P}$ is called \textit{preprimal} if $\mathbf{P}$ is
finite and $\func{Clo}(\mathbf{P})$ is a maximal clone. A \textit{preprimal
variety} is a variety generated by a preprimal algebra. After Rosenberg's
classification of maximal clones \cite{ro}; we have that a finite algebra is
preprimal if and only if its term operations are exactly the functions
preserving a relation of one of the following seven types:
  1. Permutations with cycles all the same prime length,
  2. Proper subsets,
  3 Prime-affine relations,
  4. Bounded partial orders,
  5. $h$-adic relations,
  6. Central relations $h\geq 2$,
  7. Proper, non-trivial equivalence relations.
  In \cite{kn} Knoebel studies the Pierce sheaf of the different preprimal
varieties and he asks for a description of the Pierce stalks. He solves this
problem for the cases 1.,2. and 3. and left open the remaining cases. In this
paper, using central element theory we succeeded in describing the Pierce
stalks of the cases 6. and 7..
