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So I have really difficult time finding parts. I need to swap the rear cassette to a new one because the chain and some sprockets are worn out. However I can't find ig cassettes anywhere and I don't understand any of the numbers.

The bike was bought 1999 or so and has 7 sprockets rear and 3 in front. So 21 speed. It's been well maintained so I don't want to get rid of it now. I recently bought 2 ig chains (kmc z8.1) for spares and did maintenance and replaced other bits. So I have a bit of sunk cost fallacy going on here too. I think the front sprockets need replacements soon too.

Is there a 7 speed cassette out there that is compatible with my ig chains(kmc z8.1)? Some sram, kmc or cassette maybe? Could I use hg 7 speed cassette with my ig chains? Or do I need to just suck it up and swap to hg parts? And what all hg parts would I need? Or just buy new bike?

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Every currently produced 7-speed cassette you can buy, which all have Hyperglide-style ramping, will be fine with your KMC Z8.1 and all the other currently produced Shimano, SRAM, and KMC 7/8-speed chains. You should buy whatever appropriate range 7-speed cassette you can get your hands on.

Shimano called out various incompatibilities involving HG vs IG chains. The third-party makers just say their chains work with both. (You can probably find some exception to this if you look back far enough). I've never had an issue dealing with the HG vs IG question by just putting a SRAM or KMC chain on.

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    Very much a case of perfect being the enemy of good. (Although the profits the manufacturers get by suggesting perfection is required are good... for them) – mattnz Apr 26 at 23:33
  • It's a bold claim that either IG or HG chains are perfect – ojs Apr 27 at 6:24
  • @mattnz Over the years I've grown more sympathetic to Shimano on such things. Most of the time they couldn't play it looser and still be able to stand by their products necessarily functioning as designed. It doesn't mean their word is law, but it's not evil either. – Nathan Knutson Apr 27 at 8:50

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