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I am fitting a modern Shimano 10-speed Hyperglide cassette (CS-HG81-10) to an old Shimano 8 speed hub (FH-M565). Fortunately, it is long enough (approx. 35mm) to take 10-speed.

The hub came with a spacer and I am not sure if I should use it or not. I know that when fitting 9/10 speed to an 11 speed hub you should use a spacer, but I am going up from 8 to 10!

The spacer measures 1mm. I can fit all sprockets with and without the spacer, however with the spacer the final sprocket will extend slightly over the end of the hub.

How do I know if I need a spacer or not?

  • Most 10-speed Shimano cassettes do come with a 1mm spacer that should be used, and hubs never do out of the box. Are you sure it was with the hub? What cassette did you get? – Nathan Knutson Jun 3 at 1:04
  • @Nathan Knutson I've gotten a couple SLX cassttes (CS-HG81) and a SunRace lower tier 10s cass. All new in the box and none came with a spacer. The current SLX is mounted on an XT hub (FH-756) and has a bit more play than I recall either the SunRace or SLX cassettes had on a Shimano Alivo hub laced to my other rims. I should install a spacer, huh? – Jeff Jun 3 at 5:47
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    @Jeff Sorry, road cassettes usually come with the spacer – Nathan Knutson Jun 3 at 5:55
  • @Jeff it is a Shimano CS-HG81-10 – sdgfsdh Jun 3 at 8:49
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My guess is that the spacer is for fitting 7 speed cassettes (which are narrower than 8-10 speed cassettes).

Try installing the cassette without the spacer, if you can tighten the lock ring up and it isn't bottoming out on the freehub, I think you are good.

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    I can install with and without the spacer (the final sprocket extends slightly over the edge with the spacer). It's hard to tell if it is bottoming out or not, since at 40nm it is very tight anyway. – sdgfsdh Jun 2 at 19:38
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Most Shimano 10-speed cassettes road cassettes come with a 1mm spacer. It is intended to and should be installed whenever it can fit on the freehub body.

Mountain cassettes never include or use this spacer.

For cassettes that include it, in many instances the small cog overhang past the end of the freehub body will be marginal if the 1mm spacer is left out in an installation where it will fit, potentially causing the lockring to bottom out. It's within possibility for this issue to develop or become noticeable over time, as cassette carriers and freehub bodies can have slight dimensional wear occur, particularly aluminum ones.

In the case of installations where other spacers are present, Mavic being the classic example, it should be installed in addition to the other spacers.

For the tall-spline Shimano 10-specific hubs, i.e. FH-7800 and WH-7800, the spacer will not fit and can be left out.

The spacer also exists to create cassette location interchangeability, i.e. for wheel swaps sans adjustment. In other words, the design of the various hubs and 10-speed cassettes is intended to create the same dimensional relationship between the cassette and the dropout for an 8/9/10 hub with a 10-speed cassette that has its 1mm spacer installed, an FH-7800, an 8/9/10 hub with a Tiagra CS-4700 10-speed cassette (not tall spline compatible and doesn't come with the spacer), an 11-speed hub with the 1.85mm conversion spacer plus a 10-speed cassette with its included 1mm spacer, and a Mavic hub with the 1.8mm spacer for Shimano 8/9/10 cassettes plus a 10-speed cassette with its included 1mm spacer. An 8/9/10 hub hub with a mountain 10 cassette will also put the cogs in the same place, although it probably won't work in the same bike as the road ones.

There may be some instances where leaving the spacer out on an installation where it will fit will create improper contact between the cassette and the shoulder of some freehub bodies. (Some cassette carriers have a lip for the spacer machined out).

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  • The spacer came with the rear hub (FH-M565) not the cassette. – sdgfsdh Jun 2 at 22:05
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You don't need the spacer. You are correct, it is intended for when you are using a narrower cassette. If you can torque the lock ring without the spacer it shouldn't be used. It's likely that your derailleur will have the range to suit if it worked with your 8 cog cassette, so you should be able to tune and go. If you haven't invested in a 10spd der yet, hold off and see if the old one works.

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  • Note Shifters have different pull ratios for different models and manufacturers. You cannot just mix and match and expect it to work - for instance Shimano 10 shifters won't work on an 8 Shimano Speed deraileur. – mattnz Jun 2 at 20:39
  • Some 8 speed ders have the range and work great. Ive done it. I imagine he's already got a 10 spd der for it anyway. – bradly Jun 2 at 21:08
  • @mattnz Shimano 10 speed until recently had the same ratio as 8 and 9 speed. Now there two separate 10 speed versions, but the classical one is compatible. – Vladimir F Jun 2 at 22:08

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