I've had my bike since new in 1993. I rebuild the hub bearings every so many years. I recently rebuilt the rear after letting it go for too many years and had a nagging question:

(Reference the linked image. Shimano Acera X 7spd hub.)

See the boot seal on the non-drive side of the rear? Is that supposed to be fixed to the axle spacer and rotate against the hub body?...Or is it supposed to be fixed to the hub body and rotate against the axle spacer?

The reason I ask is that it has been doing the former (rotating against the hub body) for the past 20 years or so and it's showing some undesirable wear (and the part is probably NLA). And I could swear that when I first bought the bike that seal stayed fixed to the hub and it was the end by the axle spacer that rotated.

Anyone know which is correct?

enter image description here

  • 1
    It isn't really "fixed" to either-- though usually they stay oriented with the axle. Not always though.
    – Noise
    Sep 15, 2023 at 14:56
  • By "fixed", I mean that it has to rotate and it's going to rotate/slide against one of the two surfaces. I thought the wear at the hub end of the seal indicated that I didn't lube the other end well enough and it stuck. I'm trying to get this hub to last before I'm forced to replace it or the wheel.
    – Mike
    Sep 15, 2023 at 15:04

1 Answer 1


Most of the time they remain fixed relative to the axle spacer, and the hub shell rotating against the seal is the dynamic contact.

If it's not doing that anymore, you could try cleaning the axle spacer end thoroughly and leaving it dry, then putting a few drops of oil on the seal where it plugs into the hub shell.

They do wear. Some hubs had these available as spares once upon a time - I remember seeing well-intentioned and mostly unused bags of them when I started in shops in the early 00s. The hub in the picture is a common one so finding a specimen to rob parts from for cheap is likely doable.

  • Location dependent. The only local bike shop that could have one is run by kids too young to know what things are. You know where the boots are available?
    – Mike
    Sep 15, 2023 at 15:34
  • 1
    @Mike Googling "FH-M290 ev" gives plenty of hits on the exploded view diagram, which shows that 3AC 0400 is the boot seal. Googling "Shimano 3AC 0400" gives us some hits on places it at least has been available online and shows us its the same part used on many other similar Shimano hubs, though nothing obvious in terms of an online retailer that just lists it straight up. I believe the part in question is the same one lots of 90s- and early 00s-era shops had a baggie of, so it would likely be a matter of asking one that's still around or getting a donor hub from a used place. Sep 15, 2023 at 16:16
  • 1
    It also appears 3AC 0420 is an updated version. Sep 15, 2023 at 16:21

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