Join Date: Sep 2006
Location: San Francisco
Bikes: 09 Tarmac Pro SL / Sunday BMX /
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The 42mm chain line fighure is the measurement from the center of the bb shell (centerline of frame) to the middle of the chainring, usually measured from the center of the seat tube to the center of the ring.
Now, if you were to use a phil bb with a 103mm spindle in either a 68 or 73 mm shell the chain line should be the same. as sheldon states, the cups on a 68 shell will stick out about 2.5 mm on each side. that adds up to 5mm, the exact diffefence between a 68 and 73. This means that when you install the phil 103 bb in your 73 mm shell the cups will be flush with the frame, not sticking out as they would in a 68mm bb shell. As long as you use a phil 103 mm bb axel you will be fine.
Keep in mind that you can adjust the chain line with phil wood bottom brackets to fine tune your chain line.
The reason this will work with the phill bb is because of the fact that the phil wood bb does not use a fixed cup like most other bottom brackets. if you were to use a standard cratridge bottom bracket then you would probabbly run into some problems due to the spindle being offset to one side.
disclaimer: i just got back from drinking at my friends house so this makes sense in my head. apologizes if my explanation is completely whack.