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I have a Giant STP1 (circa '05), with the Shimano derailleurs. Got a new Shimano HG40 chain to replace the old one (don't remember the brand/type), and it's skipping under load.

Derailleur? Skipping happens on most gears, but the derailleur is aligned, and straight. And shifting is precise, with no drop-out in any gear.

Wrong chain type? I don't have the original chain anymore (it rusted), but there was no skipping before, so I'm blaming the new chain. Later versions of the bike use the HG40 chain, but they use different derailleurs, and have a different cassette.

Does anyone know what the original chain was on the '05 STP1? The name Rohloff comes to mind, but I'm not sure.

Update: After reading some similar questions, I'm thinking that maybe the gears are worn out after all... But so was the old chain, and they fit together.

gearset

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    This is probably a duplicate of many questions already on this site. Short answer: you didn’t replace your cassette, and this existing one is so worn that its pitch doesn’t match the new chain. Worn (elongated) chains wear out cassettes and if you let if go too far, a new chain will skip on the worn teeth
    – Paul H
    Jan 2, 2023 at 4:05

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Your cassette is too worn to mesh with a new chain.

Here's a tooth/valley from your photo showing a lot of wear.

enter image description here

The next valley down shows even more wear. The one above, has two distinct circles rather than a straight exit line but there's a definite change in the two curves.

If you let the wear go far enough, then you might need new Jockey wheels and at least one new chainring too. Chainrings have more teeth engaged so tend to spread the load better, but still worth inspecting.

Personally, I ride my chain into the ground and then expect to do the cassette and chain together, but I'm not riding with a fancy cassette costing hundreds of dollars.

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    Even on a brand new cassette the teeth are very irregular. How exactly do you see the wear on the red-marked parts? What I would notice first, is that some valleys are extremely wide when compared to the teeth between them. Jan 2, 2023 at 10:04
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Yes your cassette is worn out, and also i wonder did you cut the new chain to correct size? Cn40 is ok for 8 speeds. There is nothing wrong with chain.

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