Join Date: May 2004
Location: Wilmington, DE
Bikes: 1984 Trek 660, 2003 Specialized Hardrock, 2004 LOOK KG386i (RIP), 2005 Iron Horse Warrior Expert, 2009 Pedal Force CX1, 2014 Islabikes CNOC 14 (son's)
Mentioned: 1 Post(s)
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Quoted: 189 Post(s)
Your concern should be whether or not the resulting chainline that you have is acceptable. Shift into the little/big combo and the big/little combo and see how straight the chain is. If the chain is slanting in the same direction (either towards or away from the bike) in both of those combos, then your bottom bracket length is not the right length. Ideally, the chain rings are centered on the cassette allowing you the best compromise of a chainline while using any gear combination.
Trek may have used a 107 BB and that may be a compromise versus a 110 BB or it could be that your LBS made the compromise. Either might have made their choice simply based on what they had in stock. It's very possible that both lengths will give you basically the same chainline compromise just that one length has the compromise slightly shifted towards the bike and the other has it away from the bike. After all, the different is only 1-1.5mm on the crank side.
Basically, if the bike shifts ok using the normal gear combos (avoiding the little/little and the big/big) I wouldn't worry about it.