Join Date: Feb 2007
Location: Mid Willamette Valley, Orygun
Bikes: 86 RockHopper,2008 Specialized Globe. Both upgraded to 9 speeds.
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The 118mm dimension is the spindle length of the Bottom Bracket (BB).
A new BB is only about $15-30, depending on quality, so even if your new cranks require a different length, it's not that big of a deal. Especially if you are buying expensive cranks!
When you search for cranks, they often specify the required BB length.
The whole idea is to keep the chain line running "straight" between the chain rings and the rear cogs.
That's obviously only possible in a couple ring/gear combinations.
In my riding, I tend to use the middle ring and outer gears. I purposely selected a slightly long spindle length because it gives me an "average" better chain line for the gears I use the most.
Keep in mind, the rear cogs are spaced 5mm apart. Being off 3 or so mm really shouldn't make much difference, assuming you have a "perfect" chain line now.