Hello! I am trying to upgrade my bike’s Octalink BB 118 mm to a 105 5600 BB and crankset. The problem is I can not make the plastic pin of 105 left leg fit the hole. I think the axle is too short or BB is too wide? Thanks in advance.!
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Check your shell width. Shimano road cranks only work with 68mm shells and require 'road' version of the Hollowtech BB. (MTB cranks / MTB BBs work with 68 and 73mm shells.) I think it's likely you have a 73mm shell.
See this answer for details on the difference between road and MTB BBs.
Update: just looked at the pic of your old BB and it is in fact marked '73'.