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So currently I have Shimano SIS downtube shifter (6 speed) with indexing, a 6 speed freewheel, and Tourney TX for the rear deraileur, they index properly.

If I want to swap the RD with Shimano Claris, will it still index properly?

Why? because I'm planning to upgrade the wheelset and shifter in the future to end up with full Claris, but I want to do it incrementally.

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Proper indexing of the derailleur cage relies on two things:

  1. Length of cable pulled per click of the shifter.
  2. Derailleur actuation ratio (how many mm the derailleur cage moves per mm of cable pull).

If the derailleur is swapped for another with the same actuation ratio the indexing will be unchanged.

Shimano used an actuation ratio of 1.7 from 6 speed to 9 speed mountain and 10 speed road. (Source: Arts Cyclery Blog - Drivetrain Compatibility).

So yes, the 8 speed Claris derailleur should work with the 6 speed shifter and 6 speed freewheel.

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Very likely yes. The Rear Derailleur mechanism has no indexing functions, and it just reacts to pull from the shifter mechanism.

All the indexing is done by the shifter on the bars/downtube.

The only thing that could mess you up is if the amount of cable pulled per click is different, but 6, 7, and 8 speed cassettes all use a 5mm spacing. If you were doing something with a 9 speed, this would be an issue.

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  • Yes, I'm ignoring the older indexing systems, its very unlikely to be one of them. – Criggie Apr 15 '18 at 11:07

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