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Old 12-07-19, 09:15 AM
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rccardr 
aka: Dr. Cannondale
 
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Might be easier and less expensive in the long run to pick up an 80's Shimano Deore or 6206 triple crankset, which is 110 for the big and middle rings, and 74 bcd for the inner ring. Use a chain guard or cut down 110bcd inner ring as a spacer where the outer ring used to go, then put your large 110bcd ring where the middle ring used to go, and your small 74bcd ring in the granny position. Set it up on the BB just like a triple -a 118mm asymmetrical spindle in a cup & cone bottom bracket will do nicely and not make the NDS arm stick out way far, although a modern 120 symmetric might also do the trick- so that the inner ring & associated mounting bolts miss the chain stay by a couple of mm. With the downtube shifters on your Centurion, should shift beautifully.

You'll also have a marvelous ring selection to choose from: 52 to 34 for the big ring, and as small as 24 for the granny. Plus, because the mentioned crank arms are from the same design period as what's on the bike, it won't look out of place.
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