Clauson (1972)
An etimological dictionary of pre-thirteenth-century Turkish
p. 823
sekkiz
‘eight’; like ottuz, ekki:, q.v., and three other numerals it originally had a medial double consonant, but this is seldom written and in many languages not pronounced. S.i.a.m.l.g. with some phonetic changes (-k-/-g-; -z/-s). Only(?) SE Türki sekkiz; SC Uzb. sakkiz preserve the -kk-. Türkü viii sekiz (for sekkiz) is common; viii ff. including Man. and Yen. ditto: Uyğ. viii ff. Man.-A: Man.: Bud.: Civ. ditto: O. Kır. ix ff. ditto: Xak. xi sekiz the number ‘eight’; it is an abbreviation (taxfīf) of sekkiz Kaş. I 365; a.o. I 437 (seksö:n): xiii(?) Tef. sekiz 266: xiv Muh. ‘eight’ sekkiz Mel. 81, 8; sekiz Rif. 186: Çağ. xv ff. sekiz (‘with -k-’) sekiz Vel. 288; sekiz (spelt) ‘eight’; also pronounced sekkiz San. 255r. 10; (on the -kk- see 20v. 9ff.): Kom. xiv ‘eight’ segiz (sic) CCG; Gr.: Kıp. xiii ‘eight’ sekiz Hou. 22, 8: xiv ditto İd. 53; sekkiz Bul. 12, 11: xv sekiz Kav. 65, 7; Tuh. 60b. 7.