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I need to replace a Shimano FX-TX70 crankset, "DualSisIndex", 42-34-24.

However, I'm quite lost on the Shimano website, and I'm not so sure which part to choose as a similar replacement...

Would someone be able to help?

Thanks

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  • Possible duplicate of bicycles.stackexchange.com/questions/54341/… (Different product number but appears to have the same upgrade path.) – DavidW Jul 31 '19 at 17:27
  • You need a replacement crankset that takes the same bottom bracket - including axle length - or replace it too. I couldn't find the length for FC-TX70, can you measure it? – ojs Jul 31 '19 at 18:10
  • Well, that's where I'm not so sure (I'm a real newbie with bikes) I initially bought a random crankset, as I expected a standard. And the one I bought gets too far away from the frame, and it is not compatible with gear system. I learned the hard way that there's no standard :) And I don't really know what to measure exactly... (And I arrived here...) – Yako Aug 1 '19 at 10:07
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You can nominally replace the TX70 crank with any 3x7 speed, square taper axle crank with the same chainring sizes (or different sizes if you want a different gear ratio range).

However, you need to make sure the new crank maintains the same chainline as the TX70. (Chainline is the distance from the centerline of the frame to the middle chainring, 47.5-50 mm for a MTB triple.) Different model cranks require a different axle length for a given chainline, so the challenge is finding the crank and bottom bracket specifications and matching them up. It's often easier to replace both the crank and bottom bracket as you can select a pair that give the correct chainline.

Shimano provides an extensive archive of specifications. I found the FC-TX70 in the 2004-2005 document. It says the FC-TX80 provides a 47.5mm chainline with a BB-UN25 D-NL bottom bracket. This is the only option given so we can assume your bike has a 47.5mm chainline. Finding the specifications for the BB-UN25 we see the D-NL version has an axle length of 122.5mm. The archive specification documents are very long, but you can easily use search to find the FC-TX70 and BB-UN25 product codes.

Looking at the current crank specifications and taking the FC-TY501 crank for example (again, use search to find the specs for that crank model), that crank provides 47.5mm chainline with a BB-UN100 or BB-UN26 bottom bracket, D-NL version. Looking up the BB-UN26, the D-NL version has a axle length of 122mm.

Bingo!

You can obviously go through the same process with the other current Shimano cranks, figure out which ones will replace the TX70 directly or will require a new BB.

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  • @DavidW As product recommendations are off-topic I'm not going to specifically recommend the FC-TY50, although obviously I have pointed out it is a drop in replacement. – Argenti Apparatus Jul 31 '19 at 21:49
  • Ah, I understand. Thanks for clarifying. – DavidW Jul 31 '19 at 21:55
  • Thank you very much for your detailed answer, and tutorial approach. As you understood, I'm new here, and I'm discovering a whole bunch of concepts and technical terms :) I followed your process, and could identify two different models with the right 47.5mm chainline (FC-TY501 & FC-TYT301), with the same bottom brackets that fit the length of the one I own (supposably the BB-UN25). So I guess I can purchase one of them. Thanks for your help! :) Thank you very – Yako Aug 1 '19 at 11:15

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