My daughter's Trek came with a 6-speed Shimano Revoshift SIS Index. It's a new bike. She can shift to higher gears easily but does not have the grip strength to shift back
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How to replace shifter cable on Shimano revoshifter - YouTube
How to replace cable on a budget Shimano Revoshift shifter. | budget, Shimano Inc. | Shimano-MTB Revoshift is a budget twister shifter https://ebay.us/f5Adaf and the shifting cable can be replaced or installed
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