There are a couple of examples in which a long [ō] became [ū] in Ilkorin, a peculiar development because the Comparative Tables indicate that primitive [ō] was usually preserved (PE19/25). Helge Fauskanger noted this aberrant development for ᴹ✶dōmi > Ilk. dûm, but did not speculate on a general rule (AL-Ilkorin/dûm). After Fauskanger’s work, a second example was published in the Addenda and Corrigenda to The Etymologies: ᴹ✶Górōmē > Dor. Grûm from a deleted entry (EtyAC/GÓROM).

The change precedes the consonant [m] in both examples, so it is logical to assume that this is the situation leading [ō] to become [ū]. There are other phonetic rules in Ilkorin that associate [u] and [m]: in the development of vowels from syllabic final [l], [r], [n], [m], the vowel that developed was usually [o] (-Cl > -Col; -Cr > -Cor; -Cn > -Con) but in the case of [m] was [u] (-Cm > -Cum).

This change must have occurred before [ā] became [ō], because the change of [ō] to [ū] did not apply to ᴹ✶kwāme > Ilk. côm (Ety/KWAM). It is possible this development occurred with short [ŭ], but there are no attested examples. Helge Fauskanger suggested that may also have occurred before other nasals such as [n] (AL-Ilkorin/thôn).