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8 speed cassette spacing

Old 07-17-21, 11:21 AM
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8 speed cassette spacing

I picked up a used Shimano Hyperglide XTR CS-900 12-28 cassette. I plan to put on road bike with Dura Ace RD-7403.
According to Sheldon Brown table, should be 1.8mm thick, 3mm spacing, and 4.8mm ctc spacing.

The 4 biggest cogs (18-28) are on a spider, and match the table dimensions. They have stamp “Q R”. Same for first loose cog (16T) and spacer.

But three smallest cogs (12,13, 14T) do not match. They are 2mm thick.
The 12T has expected 3mm integrated spacer.
The 13T has ~3.2mm integrated spacer.
These 3 have “Q” stamp.
These 3 don’t line up exactly with any of the Shimano dimensions in Sheldon table, 7 or 8 speed, so not sure what to make of it.

Do the smallest cogs sometimes have different dimensions, or should all cog in cassette be same?
I am wondering are the three smallest cogs original from the cassette, or a mash-up pulled from another cassette?
Will these work on 8 speed or cause shift problems?

Thanks in advance for any insight.

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Old 07-17-21, 06:10 PM
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Ok I can continue talking to myself and post my findings 😀

I searched a bunch of similar, but not exact, part images on the web. They look similar to mine, so that tended to make me think mine is still a complete original cassette. I also measured my Dura Ace 7400, and it had similar thickness (2mm on smaller cog, 1.8 on larger). Also second cog had thicker spacing… go figure.

So I installed the CS-900, didn’t even need to adjust anything (not even B screw). Shifts great!
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Old 07-18-21, 12:26 AM
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The “Q” stamping indicate those that are part of the original cassette. The finish that looks common to all of them suggests that they are the original set. You’ll find that one in the table within this article from the Sheldon Brown compendium. It sounds like you’d already discovered that resource. Remember that DA 7400 stuff had some unique characteristics.

Generally the spacing is pretty uniform, but as you’ve discovered, it can vary, especially with the outermost cogs. If the shifts are good, then you’re golden!
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Old 07-18-21, 10:11 AM
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I believe this is the right techdoc for your cassette:

The Sheldon tables imply that cassettes have uniform spacing, but as you're finding out, Shimano and others were willing to make things slightly uneven for better shifting. My 7-speed cassettes exhibit the same thing -- slightly wider spacing as you get to the small end of the cassette. Shimano hoped that by riveting a bunch of it together and making the remaining cogs and spacers unique, there would be less chance of spoiling this arrangement.
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