The labdane-related diterpenoids (LRDs) are an important superfamily of natural products whose structural diversity critically depends on the hydrocarbon skeletal structures generated, in large part, by class I diterpene synthases. In the plant kingdom, where the LRDs are predominantly found, the relevant class I diterpene synthases are clearly derived from the ent-kaurene synthase (KS) required in all land plants for phytohormone biosynthesis and, hence, are often termed KS-like (KSL). Previous work, initiated by the distinct function of two alleles of a KSL from rice, OsKSL5, identified a single residue switch with a profound effect on not only OsKSL5 product outcome but also that of land plant KSs more broadly, specifically, replacement of a key isoleucine with threonine, which interrupts formation of the tetracyclic ent-isokaurene at the tricyclic stage, leading to production of ent-pimaradiene instead. Here, further studies of these alleles led to discovery of another, nearby residue that tunes product outcome. Substitution for this newly identified residue is additionally shown to exert an epistatic effect in KSs, altering product distribution only if combined with replacement of the key isoleucine. On the other hand, this pair of residues was found to exert additive effects on the product outcome mediated by distantly related KSLs from the eudicot castor bean. Accordingly, it was possible to use a rational combination of substitutions for this pair of residues to engineer significantly increased (dominant) selectivity for novel 8α-hydroxy-ent-pimar-15-ene product outcome in the KS from the dicot Arabidopsis thaliana, demonstrating the utility of these results.