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8 speed cassette spacing

Old 07-17-21, 11:21 AM
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Steel1
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8 speed cassette spacing

Hi,
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.

https://www.sheldonbrown.com/k8.shtml

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: https://si.shimano.com/api/publish/s...00-8Q-1206.pdf

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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Old 08-12-21, 02:45 PM
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This is pretty insightful. I've always thought the 1st/2nd position spacing was wider because 7 and 8 speed has the added sprocket washer (per Shimano diagram). I never realized it continued to the 2nd/3rd... only for the 8 speed? It also explains why some people are able to run 8 speed shifters with 7 speed cassettes fairly well.

Since .1mm = .004", going from 3.00 to 3.20 is only .008" and surely within the Centeron pulley movement. But that can still pose an issue.

I've gone way outside the box on fitting an 8 speed cassette into my 126mm dropout Cannondale. Intentionally using the wrong 9 speed freehub body on a FH-7700 to make the cassette closer to the spokes but having to add .8mm spacer behind the cassette and still keeping the OLD between 127mm and 128mm.

Everything worked; keep in mind I'm running a 14-16-18-20-23-26-30-36 cassette and with the .8mm behind it, there is a challenge to get everything on the freehub body. But the 2nd position, downtube shifter, did not have a positive firm click and sometimes needed some additional nudging, or it was a bit too noisy. Dialing in the 2nd position was detrimental to the rest of the cassette. I started to expand the spacing between the 2nd and 3rd position and that helped. But being space limited, I couldn't go too far. The OP's spacing measurements helped me understand how much space I needed.

The result is a pretty whacked out cassette that is runs very smooth:
14t - 11 speed Ultegra 1st position with oversized integrated spacer sanded down to fit.
16t - 10 speed XT with a 3.25mm spacer behind it.
18t - 8 speed Sunrace standard spacing.
20t, 23t, 26t, 30t - 8 speed Shimano standard 3.0mm, except 23t to 26t is 3.08mm
36t - 9 speed Sunrace standard 3.00mm spacer.

John

Last edited by 70sSanO; 08-12-21 at 02:49 PM.
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