Are you aware that there were two Octalink interfaces, V-1 and V-2? V-1 was used with road cranks (105, Ultegra, Dura Ace) and XTR MTB cranks. V-2 was uses with the other MTB cranks. If the recommended spindle was 121 mm it has to be a V-2 since, AFAIK, V-1 only came in 109 mm (double) and 118 mm (triple) lengths. Be sure the crank is matched to the correct Octalink type.