We investigate completions of partial combinatory algebras (pcas), in particular of Kleene's second model $\mathcal{K}_2$ and generalizations thereof. We consider weak and strong notions of embeddability and completion that have been studied before in the literature. It is known that every countable pca can be weakly embedded into $\mathcal{K}_2$, and we generalize this to arbitrary cardinalities by considering generalizations of $\mathcal{K}_2$ for larger cardinals. This emphasizes the central role of $\mathcal{K}_2$ in the study of pcas. We also show that $\mathcal{K}_2$ and its generalizations have strong completions.