This paper examines sound substitutions in Finnic loan words from Proto-and Ancient Scandinavian. Correlations are studied between Finnish/Estonian ai, äi, ei, and õi and eastern-Scandinavian descendants of the Proto-Scandinavian diphthong ai, which developed over ɛi into the Old Swedish ē. On the basis of the observed correlations, substitution practices and their chronologies are reconstructed. A further aim of the paper is to seek independent evidence for the quality of the diphthong during different stages of eastern Scandinavian development. This subject is intertwined through chronology with the borrowing of Finnish napakaira meaning 'bradawl, auger', namely with determining the form of the Scandinavian loan original *naƀaǥaiza-> Old-Icelandic nafarr (in particular *[z] > [r]) at the time of borrowing. It is shown that within a material of some 80 potentially borrowed pre Viking Age Proto-Finnic stems there is not a single ascertained case where a descendant of this Scandinavian diphthong would have been substituted by *ei, whereas the very few possible cases of *äi cannot be judged to testify to a fronted quality in the original. Substitution practices using back stems, which were subject to Finnic palatal vowel harmony, persisted for a surprisingly long time. In parallel with the principal ɑi → *ɑi substitution, one with *ëi ~ [ɤ̞ i] is used in 2–4 ascertained cases. This substitution may testifiy to some variation or modification of the Proto-Scandinavian diphthong.